There are many schools of thought in islam: Hanafi, Zaidi, Hanbali, Maliki, to name just a few
But how were these schools created?
I know that many of them had a founder, Hanbal created the Hanbali school for example, but I still don't know how that works, I have so many questions
For example, let's say you wake up one day and decide to create your own school. Presumably you already belonged to another school, how will those people react to you creating a new one? Then there is the philosophy, these schools have positions on every philosophical and theological debate. Do you need to already have defined your positions on every issue? What if you change your mind? How do people join your school of thought? Was there a formal ceremony? Or did people just say: "Yeah, I agree with this guy." What if your school of thought doesn't get traction?
There are many schools of thought in islam: Hanafi, Zaidi, Hanbali, Maliki, to name just a few
a helpful starting point would be to parse out these different schools:
Hanafi, Hanbali & Maliki would all be madhhabs or schools of jurisprudence. Zaidi, on the other hand, would be a theological branch of the Shi'a. Schools of jurisprudence and schools of theology are quite different things.
In terms of the schools of jurisprudence, we see that the four big ones that are still around today (as well as extant ones, for that matter) are all named after an important scholar, like Ahmad ibn Hanbal (referred to as Ahmad or ibn Hanbal). These were famous scholars of jurisprudence in their times and as such they amassed students. Their students did a few important things: 1) passed forward and/or recorded their legal verdicts, 2) learned their teacher's methodology for arriving at those verdicts, 3) codifying that methodology, and 4) passing it forward. A lot of these differences in methodologies deal with things like how to deal with apparent contradictions and clashes between different sources of evidence and the relative weight of different sources of evidence.
Anyways, having students that learned the teacher's methodology and carried it forward was how these schools formed. We know of other great scholars around the same time like al-Layth or ibn Jarir whose verdicts are recorded and still written about, but their methodology for arriving at those verdicts has died out due to lack of students or it not having been recorded or whatever else may have been the case.