I have been experimenting with SPO for a while now and only recently discovered that the claims format for an authenticated user is like this: "i:0#.f|membership|username@domain.com"
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On-Premises I know that the "f" denotes that forms authentication is being used and w is used to denote where the source directory is AD.
So, does SPO actually use forms authentication although there obviously exists the concept of Azure AD that we authenticate against?