I have seen this useful feature in several Windows File Explorer clones – the flat view. Basically the feature allows you to view all the files in the current directory and sub-directories, side-by-side, as if they were all in the same directory. Useful when trying to compare/find files. I don't think it exists in Finder, but what about the various Finder replacements?
Flat view in Finder – does it exist
finder
Best Answer
Spotlight can, if you just search for "-kind:folder". It also shows the files. It looks like a bug to me, but it works.
Found here.