[ywei3410]

I think it's worth noting that when we talk about a system operating on a set of components, it literally means a function which either reads and creates new components or entities or mutates existing components.

Most "things" don't exist in ECS. There's no singular "Mario" or "gravity". If you feel like you ought to split it into multiple identifiers for your systems (for example, treating each part of Mario like a different sprite), then you do so.

The hardest part of ECS IMO, is that dependencies between systems are super loosely defined since it's meant to be modular and the dependencies can only be tracked by components. The semantics of ECS systems are also highly technical (do you have hard sync points, how do you deal with recursive entities etc...) which means it can be quite fiddly to work with.