The System-of-interest depends on who is looking for it
The target system is defined subjectively, as a "system in the eyes of the beholder," so the question of defining the "system you want to create" starts with attention to who exactly "you" are: how many of you there are, what interests you have. Systems thinking is collective/group thinking, allowing large teams to negotiate in projects with millions of individual details in target systems, keeping the attention of different roles on their common goal, in order to take care of the details of subsystems to individual members of each team without losing focus on the system as a whole.
There are no strict rules, algorithms, sequential thinking steps, or survey orders, or case typologies (this is an entrepreneurial/creative/research task! There is no algorithm for entrepreneurship/creativity/research, only generating conjectures and critically rejecting them!). In any case, the decision regarding the choice of the target system will involve a wide variety of people in different roles, not just team members but also external roles. If you have a project that you can do alone, you won't need elaborate system thinking, snippets of it will be enough. But if the project is complex and involves a group, decision-making will involve a complex negotiation process, and agreement on the target system will need to be reached as the project progresses.
Usually, the target system cannot be defined by sitting at your workstation and reflecting: it requires not only reading various documents and knowing your subject area well, but also meeting and talking to many people, sometimes conducting experiments, including implementing models, prototypes. Do not confuse "our system" (MySystem, OurSystem, engineered system, "my" personal, or "our little team's" system) and the target system: without the target system, system thinking for our system will not be successful, and therefore our system will not be successful — we will be happy, our interests will be taken into account, but success is primarily determined by the success for external roles, not the success of our little team's roles! We will create "our system," but money is usually paid for the target system: without a link from our system to the target — there will be no payment for the work, and the project for our system may not be started immediately. Similar reasoning will apply in cases where there is no monetary payment, but there are other goals to be pursued. For example, if you work on a political project and are involved in a small part, investing your own labor resources and expecting nondenominational results from the realization of our system and eventually the target system.
Every project is unique (a combination of the project roles making it! Not just the system the project is creating), every situation in an activity is unique, so nothing can be memorized and applied later in the same form as memorized. You cannot enter the same river twice, you cannot make the same system twice in the same way. Even if you (as a team!) are doing something very similar for the second time in a row, you gain experience compared to the first time, you are better aware of risks than the first time, have more information — based on this experience, you can make completely different decisions, organize work differently. "Remember, this is how it is always done" is not about system thinking; system thinking draws attention to important objects in thinking, but does not negate thinking itself. You will have to get your head out of your pocket and think in every project: system thinking helps to ask questions, but you still have to look for answers, have conversations, and make decisions.
And once again, considerations regarding the definition of the target system (like all other system design considerations) are usually made "in classes," with system types, not with individual systems, even though you work in a unique situation. Regardless of the uniqueness of projects and systems, there is still an attempt to transfer as much knowledge as possible from project to project, accumulate knowledge, and reuse knowledge.
All systems are in the eyes of the beholders, so you need to identify and negotiate with the team and external project roles on the target system, as well as the supra system with systems in the near and far environment, and subsystems, and with various system creation systems (including the team as one of the creation systems) throughout the creation chain, and also about our system in the whole set of these project systems.
Of course, each project participant (and there may be many interconnected projects here, so participants from other projects as well) has their own opinion on this matter, different from others. And project participants can also change their minds as agreements are reached (agreements are precisely about this: about changing opinions). What to do?! Continuously negotiate. Systems are created collectively, and agreements are reached in these collectives. Systems thinking helps to determine what to negotiate on. First and foremost — about what exactly we are doing: which system is the target system, i.e. the success of which system will be key to the project. No agreement on the target system — we don't know what to do, there is no project.
Why is it so difficult to define a system? Because in life, there may not yet be a convenient definition of the system level on which the system is defined. For example, if you try to propose a new way of building a wall with large blocks, but at that moment only brick buildings are being constructed (and there is no concept of a "wall," only bricks). We used this example when talking about systems thinking, which helps to deal with complexity: there are no meaningful operations with bricks in the building, but the concept of a "wall" may not be formed yet, so it is not being worked on! The idea of defining a wall and its alternative construction can be very, very fresh!
One of the real cases: it was necessary to create a system for transporting aircraft from an airfield to the manufacturing plant for repairs. Somewhere, it was necessary to enlarge the gate, somewhere to add fixtures to the railcar, somewhere to construct an additional ramp. However, all these tasks were solved as separate projects, "as they come up," they did not have common financing (and money was not allocated all at once but separately for each project, and it was necessary to orally explain each time why it was necessary to spend a certain sum and perform a certain work, there was no common list of works). There was no understanding that all this needed to be done for the efficient transportation system to be established, there was no project, no project manager, there was only a person concerned with organizing the aircraft transportation as a project (although formally the project did not exist). As soon as it was realized that these were just different parts of the aircraft transportation system, and if one of these projects was not completed, the planes would not travel from the airfield to the plant, repairs would not happen — everything miraculously fell into place, a project manager was appointed, funding was allocated, external project roles were found.
In dance, the level of muscles and bones is not the level at which we can talk about how to build dance movements. And the body is too high a level, you move part by part, and you perceive it from the inside. It was necessary to find a previously missed system level: muscle bands, proportional to the human body, perceived from the inside as such a special functional organ, the construction of which in different poses was performed by different bones and muscles. This functional organ is present in the consciousness of the dancer (and not only the dancer: we are talking about a general model) as one object that is worked with. The explanations become shorter just as when there is a wall between the levels of bricks and the house, the presence of a transport system between individual clever devices in the aircraft's path and the "aircraft repair systems"::supra system.
In one of the projects, the founders/entrepreneurs of the company collected separate fractions of oil refining waste by rail transport to large coastal storage facilities. Then they combined these fractions into one large batch/lot (a batch for trading, for a deal, for processing) and sent it by tanker to the plant (the plant did not accept small batches of raw materials), where it was processed into a very expensive product. What was their target system? What was the secret of their business? They created a lot (since the plant did not accept the initial oil fraction for processing unless it was of a certain size). As soon as the "lot" was realized as the target system, the company launched several more projects following the same scheme. Without this, the entire enterprise seemed like a random strategic/entrepreneurial/visionary find, a one-time project.
At the beginning of a project, the target system is not in focus. It is usually mentioned in the first five minutes of the conversation, sometimes even named, but people do not realize that this is precisely what they are looking for. But when this system is found (attention is paid to it, it is named and explicitly introduced in the project as the target system that everyone agrees on), life becomes much, much simpler.
Again: there are no algorithms, recipes, life hacks for defining the target system, it is exploratory conjectures/"entrepreneurial hypotheses"/non-guaranteed visionary thinking, creativity that cannot be done "by an algorithm," "by a recipe," "by a life hack," "by a procedure," or in any other way "by rules." On the contrary: there are many ways to criticize the proposed solution, and you need to implement (understanding that it is all just a hypothesis) the best hypothesis that has withstood criticism. This is detailed in research methods (based on the best explanatory theories of how they are structured, the development of ideas of Popperian evolutionary epistemology) and rationality (based on the best explanatory theories of how to build actions in conditions of an uncertain future) from the knowledge stack.
Therefore, you must express based on your intuition (possibly collective! Well-organized joint reasoning of many agents, supported by computer modeling!) conjectures about the target system and then develop from this conjecture systems creation and criticize these conjectures by subjecting them to numerous heuristics provided in our systems thinking course, methodology course, systems engineering course, and practical courses (e.g., system management as a practical course). Just a few checks in a fairly extensive checklist: the system must be physical, the system must be considered primarily as a part of the supra system at the moment of operation, the total cost of owning the system must be less than the assessment of the benefit of its operation. This checklist can be continued and continued, using all the hints on criticism and error searching from our course: if all of this is found to be met, you (likely! This is entrepreneurship/creativity/research/hypothesis! Nothing can be guaranteed!) have found the target system.