Functionally, an Entity is an ID. A Component is Data. A System is a function that operates on a set of Components.
In a sense "that's it". Having the hard separation between data (components) and code (functions) leads to some interesting consequences. For example the ability to more easily multi-thread System updates because each System clearly defines what Component type it needs const or mutable access to.
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.