When dealing with POP account, the email app will download messages, whether the default or not. Yes, there are controls, one of which is to check for new messages at startup.

Thank for that information. So as long as the Thunderbird settings are "check for new messages at startup", and perhaps "automatically download new emails", all my outlook.com emails will download to Thunderbird, even though it is not the default app? I think I no longer know what it means for an email app to be the default or not. Can you clarify that?

The default client is a term for the operating system. If you are on a webpage that has an email link, Windows will open the default email client.

Thank you very much for this help. I am going to mark my original question as solved above. My revised understanding is that a given unread email on the server will go to whichever email client next goes to that server. So the protections are (1) do not open secondary email POP3 clients being tested without this awareness, and/or (2) have settings declining any automatic downloading on secondary email clients. Apparently the developers want emails downloaded right after sign-up, which may have spooked me away from the T-bird mail app, as an unwanted surprise.

That initial setting is common across email clients. I advise people to check settings for an account before making assumptions on what the settings are. Your revised understanding is correct. I wish you well. Thank you for your feedback.