The one thing I'd disagree with is the idea that "X should check out Y if they want to learn about magic systems."
Only in that, people should check out X if you want to learn about this
specific brand of magic system.
For instance, we can look at Stands from JoJo. It is a beloved setup due to how the story incorporates it. But, as a magic system, its base components are very different. It functions in a manner closer to art due to the individualistic nature of each power and the way to gain powers is either genetically if you're related to a stand user or through getting stabbed by a specific arrow, which falls into physical initiation.
The series itself is geared to combat with stands as manifestations of someone's soul, so the actual stands are personal abilities. This is what dictates rules and limitations such as if you hurt a stand, you can hurt the user. This limitation wouldn't exist without the understanding of that personal ability factor.
A good example is if stands weren't personal abilities, but familiars. This would create different limitations and logic for why said limitation exists.
For example, it could operate closer to Chainsaw Man's system, where the users create contracts with devils. That's a negotiation-based system that draws power from spirits and is the source of power. The spirit relationship creates a different dynamic since it means having to give something to the devil in exchange for their power.
The devils, meanwhile, work on their own magic system within that, with it being a natural ability of a specific devil, but they're shaped by fear and what they're perceived to be able to do with regard to that specific fear. Their source, thus, is a subjective reality. They can just act on things, in what seems to be a cookbook set up with what they do (I've not gotten far enough to know for certain), but otherwise, they're those beings on the other half of a negotiation system. And the fact they don't need worship is why they're not a divine entity.
Regardless, every system can work differently and still be equally valid as a magic system. And what system is needed depends on the story's demands. This is why knowing the barebone components and what someone is trying to do with their story is so important before fleshing it out. Just being told to "look at X" can lead people astray and down unhelpful rabbit holes with ease. "Look at X" is what follows after someone has decided on the system they're building.
That's actually one of the reasons I made all these to help people.
I feel it's more practical to have an overview.
