Just wanted to note that proxies are no longer available in newer versions of Blender. They’re fully deprecated as of 3.0, and removed entirely in 3.1 (per the Blender Manual), in case you’re using a newer version and can’t find “Make Proxy.”
Library Overrides appear to now be stable instead of experimental as he mentions in the next video and some of the strange bugginess and crashing he was experiencing there look like it’s fixed! 