That’s my understanding too.
Think of super Mario.
Entity is Mario, the koopa troopers, the coins, the clouds, the floor tiles, the spikes, the mushrooms. All the proper nouns.
Component is the XY location in the world, ability to jump, sprite image, the height, are they affected by gravity. All the adjectives.
System is gravity - all entities with weight drop at the same rate until they hit ground. Another system may be the collision mechanic - if a mario entity lands on a koopa entity, the Koopa dies and some sound effects plays.
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.
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.
Entity is Mario, the koopa troopers, the coins, the clouds, the floor tiles, the spikes, the mushrooms. All the proper nouns.
Component is the XY location in the world, ability to jump, sprite image, the height, are they affected by gravity. All the adjectives.
System is gravity - all entities with weight drop at the same rate until they hit ground. Another system may be the collision mechanic - if a mario entity lands on a koopa entity, the Koopa dies and some sound effects plays.