Is it possible for a different (external) user to add files to my Google MyDrive (I assume: yes, but haven’t tried it myself yet). What requirements would be necessary for this?
A user can put files in your My Drive area if you invite them to a folder in your My Drive, and give them editor rights there. Commenters or viewers cannot add files. Also, permissions are always limited to the folder I’ve given them to. To gain ownership, you can make a copy - though you can also transfer ownership.
More about Google Workspace on our webpage.
Only if you’ll add someone’s email to your account at MyDrive and let them to edit.