Yeah spawn chunks are semi loaded, then each player loads a radius of 11 chunks(441 chunks). Chunkloaders loads the number of chunks you set them to ofc.No. Servers load original spawn, a certain number of chunks around players, and chunks loaded by chunkloaders. There might be a small area in each dimension loaded too, but don't quote me on that.
Dimensions will not be kept loaded(and should not be kept loaded by chunkloaders unless you need to) when players leave them and there are no chunkloaders present. In this case the dimension gets saved and unloaded from the server/pc, freeing up resources.
If you are curious about it, you can(if Chickenchunks are installed) open chatwindow and type in /chunkloaders and hit enter. A popup window will open showing all loaded chunks in the different dimensions.