If I invite another external user to “share” a file on my MyDrive: can it be any mail address, or just Gmail, or does the address need to be verified with Google in some way?
Basically, you can also share files with email addresses that do not have a Google account. Google calls this visitor sharing. A visitor has to verify himself regularly. If you want to prevent this, you can set this in the Admin Console; either as a modified Trust Rule or in the Sharing Settings of Google Drive.
See also: Restrict sharing options on Drive files - Google Workspace Learning Center
More about Google Workspace on our webpage.