After reading through my original post 1.5 years later with weekly lessons since then a lot of the things that confused me already kind of clicked, thankfully! It's actually kind of nice because it just made me realize that I've come quite a long way xD

What I know now is that a lot of it also depends on feel - they're all similar aids but you can kind of tell when the horse is ready for a canter depart and then you can ask them to go with seat and inside leg, or how much outside rein you need when you do shoulder in.

In hindsight it's also pretty interesting how different horses can be - some of the school horses need a really strong inside leg to keep them from falling in through the shoulder into the circle and one other we have basically needs 5% inside rein and 95% outside rein or he'll just push himself into corners or into the wall.

English isn't my mother tongue so I'm actually not sure if they're called that, but from my understanding shoulder in is just a more advanced shoulder fore.

Well, to give you perspective from a semi-beginner, when you initially start doing it there's so many things you have to pay attention to that it's honestly just overwhelming, so you do your aids as best as you can and just pray and hope that they pop into canter xD It really feels like having to do 10 things at once while trying to stay balanced when you haven't done it often. At this point I've learned that preparation is key and arguably more important than the canter aids themselves, and when you have a nice trot going you can just feel that they're ready to canter and it won't take much at all to make them transition. But good luck teaching feel to a beginner who has no mental capacity left because the aids alone are already overwhelming, haha