“One example that I use often — the Photos app on Mac has a built-in editing interface, but also supports extensions that allow third-party apps (e.g. Affinity Photo and Pixelmator) to take over that whole UI and present their editing UI inside the Photos app.”
This sounds a bit like what you are after.
“One example that I use often — the Photos app on Mac has a built-in editing interface, but also supports extensions that allow third-party apps (e.g. Affinity Photo and Pixelmator) to take over that whole UI and present their editing UI inside the Photos app.”