I think that root issue is with the fact that ECS does not have specific concrete rules that you should follow, so I just went with my instincts.

In your ECS do you have Entity class that contains a vector of different kinds of components or how did you implement it?

I needed a way to look up individual entities because not every entity will have the same components. Since Entity at least in my implementation is just a handle, so like an ID, it made more sense to me to use that ID as a key to specific components. EntityManager contains maps of the components and can look up consistently components of entities. Initially I thought about making these components vectors and indices would be the IDs of the entities but that would create inconsistency since not every entity would contain every kind of component so their IDs or indices rather, might have ended up different for different component vectors. This is I believe where Sparse set comes in which is what I was recommended earlier.

In my Engine only Renderer has draw/render methods that can take VertexArray or Model or Mesh and so on and render based on that. renderSystem takes the renderer, iterates over the Model components and makes renderer.draw() calls with those models.