Which App (Photos or iMovie) is 'best' for editing Video?

Hi - IMovie, without a shadow of doubt. That is what it is for. I would not do any editing in photos, I export the raw video to iMovie to do the non linear editing (arrange a set of clips into a video with titles/transitions etc), and then leave it in iMovie projects. You could import the result back into photos if you wanted, personally I don't see the point.

Editing video in Photos is, at best, a bolt on. Also prior to Big Sur, could only trim start and end. There are no non-linear editing capabilities in Photos

Photos can be quite good for creating video sideshows of still images. Which I then sometimes edit further in iMovie.

And finally store everything back in the Photos App?

I think they probably expect you to store and manage the finished movies in the AppleTV app. It has three categories of video but-in: Movies, TV Show and Home Movie...

I am using the TV.app for finished home video projects - when I created a film of a longer vacation or an expedition with my students. But I am having plenty of shorter video clips that I am just using in slideshows among my photos, for a bit of animation - video clips of waterfalls or animals. Some of my videoclips I have turned into Live Photos (with Into Live Pro) on my iPhone. This way I can use the Live Effects to stabilise them. The video clips are stored in Photos, so they are syncing to my iPhone and iPad and all other Macs as well.