Spotlight Search: Show ALL FILES, and exclude ALL FOLDERS
You can set up some quite complex searches using  Cmd ⌘   F  in the Finder, rather than invoking Spotlight itself.

You can define many criteria &amp; even save as a smart search for future use.
If you really want to get in-depth, you can use Spotlight's own query language, though the mechanics of that are well beyond my pay-grade.
I'll let Macworld - macOS: How to use Spotlight’s query language to create an All My Files-like feature - explain in detail.