I am looking for a macOS equivalent of the Windows command:
This command, when entered into Windows Explorer search, will display ALL the files of specific directory or hard drive, without showing the folders in the search results.
You can set up some quite complex searches using Cmd ⌘ F in the Finder, rather than invoking Spotlight itself.
You can define many criteria & 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.