Usermin is not a mail server, nor does it contain mail to be downloaded. It is a mail client, just like Thunderbird (well, not just like it, but it does support IMAP/POP, as well as direct access to local mail spools).
The server you would interact with in a Virtualmin system for retrieving mail via IMAP or POP is Dovecot. Postfix for sending.
Username should be the Username you see on the Edit Users page. In a default Virtualmin system this will be user@domain.tld. That is the username. Don’t try to guess. Look at the Edit Users page and use the Username as it is shown.
I don’t understand this question. You should use the username and password of the user whose mail you want to download. There aren’t separate passwords for Virtualmin, Usermin and Server. There are passwords for users. Every user has their own password, and it is the same password whatever they’re logging into. Virtualmin and Usermin and all of the mail services use system users and authenticate to system users.
Virtualmin, on installation, enables POP3, POP3S, IMAP, and IMAPS in Dovecot. So pretty much any protocol should work for mail retrieval, assuming you have a valid TLS certificate for your domain (Let’s Encrypt should work, though many people don’t host DNS locally and end up unable to get certs for all the names you might expect to work, including, e.g. mail). The unencrypted protocols should work regardless of certificate availability.
Similarly, Postfix is configured with STARTTLS and SMTPS protocols on the submission and smtps ports, to support most mail clients. Authentication is the same as everything else: The username and password of the relevant system user.
If getting the username and password right doesn’t solve your problem, you’re going to have to look at the log and give us the exact error you get (the client may not know anything useful to solving the problem, though if it’s a connection failed error, that’s not going to show up in the mail log on the server, and would have to be a firewall or network or DNS issue).
The mail log is probably in the journal on an Ubuntu 22.04 system. So, journalctl -fu dovecot when you try to retrieve mail or journalctl -fu postfix when you try to send. There may also be a log for saslauthd.
