Choose “browse files” or whatever similar title your version uses (on pixels it’s a + symbol)
Find downloaded MP3 file, select
If you want to edit it down first then use either an app or Audacity on your PC
You can also use a file stored to Google Drive if you wanted to
I should also add this works for notification sounds. If you want custom alerts for different apps, or different contacts, or whatever you can also do that with this system.
My wifes texts are a FFXIV Moogle and her calls are music from the same game, for example