My original 2010 structure was:
>Outlook Data File
>Inbox, Drafts, Sent, Deleted, Junk, Outbox, etc...
All in one .pst, which I exported, weeks ago.

In setting up 2021 it forced me to create a profile, and to setup an initial email account, which was one of the @frontier.com. I could find no way around this. I then added my other emails using the same Outlook automatic method, and imported my .pst file. I wound up with a 2021 folder structure of:

>emailaddress@frontier.com
>Inbox, Draft, Sent, Deleted, etc...

>2ndemailaddress@frontier.com
>Inbox, Draft, Sent, Deleted, etc...

>3rdemailaddress@mediacombb.net
>Inbox, Draft, Sent, Deleted, etc...
repeated 2 more times, EACH OF THESE EMAILS IN ITS OWN *.ost FILE. I don't find any new *.pst file.

My calendar / notifications / events are working as expected. My contacts are present, but separated into weird groupings. (They also don't auto fill when I start typing one, but that is probably just a setting?) Another issue is that because I have had to use email, my original pst file is weeks out of date, with all the newer stuff only contained in the new ost files. :-(