Desktop folders on my iMac are all numbered, why?

Check your voice control settings. Turn that off and see what happens.

If you have Spotify installed, then those folders are probably from the Spotify cache. You can relocate the Spotify cache location using the instruction in this forum post:

https://community.spotify.com/t5/Desktop-Mac/Spotify-created-a-bunch-of-mostly-empty-folders-on-my-external/m-p/4397473/highlight/true#M530143