blender library linking

The video above is a great intro to blender library linking, which lets you combine multiple blender files into a single "scene", a single blend file that links to all the others.

Another advantage of breaking a scene down into multiple files, is if you have a slow computer, you can still assemble large scenes by working on it in multiple parts, each part in its own blend file.
For example if you have two files: A.blend and B.blend.  MyScene.blend can link the collection groups from A.blend and B.blend and all those objects will always be refreshed when MyScene.blend is reloaded.  This way two artists can work at the same time on A and B.