Sharepoint site collection owners' LoginNames are claims.
After some experimenting, I've been able to create site collections with 4 different types of owners:
c:0o.c|federateddirectoryclaimprovider|<GROUP-GUID>_o: an O365 group
c:0t.c|tenant|<UNKNOWN-GUID>: "Company Administrator" in Sharepoint Admin console
i:0#.f|membership|<USER-EMAIL>: an O365 user
i:0#.w|dprodmgd<123>\_spofrm_<123456>: seems to be a system account
Are there any other claim types that can be set as a Sharepoint Online site collection owner, and how do I create them?
This is the best reference I managed to find.