A distinction that can be made is simulating an artificial scene from the user's pov vs projecting new objects within an already existing scene (that is through the lens of some device).

MR combines VR and AR, such that real world elements can interact with virtual elements (e.g., placing a virtual cat on a real table).

here's a really cool video about legendary animator glen keane about the virtual environment he incorporated while creating his art ~ https://www.youtube.com/watch?v=GSbkn6mCfXE