How to move mailboxes from old Mac to new

Old: 2015 iMac running Mojave

New: 2023 M2 Mini running Sonoma

I used Migration Assistant and it did mostly a good job. But I was missing a number of mailboxes I created in Apple Mail over the years. I thought at first it did an incomplete migration of the mailboxes but now I realize it only migrated mailboxes I had created online in Outlook and the M.A. stuck them into Apple Mail. I was forced to use Outlook online because my ISP, Godaddy, started using Microsoft Exchange and when my emails started bouncing because my OS wasn't up to date enough to have the new modern email authentication, I had to start using Outlook online, which I detest. That was one reason why I got rid of my perfectly good 2015 iMac and bought a new Mini, so that I could forget about Outlook and do all my email from Apple Mail in Sonoma. Let me say here that I do not understand the ins and outs of Microsoft Exchange. Suddenly my emails were synched across my devices, which I did not want. But that's another issue. As stated above, I now realize that the only mailboxes that were migrated were the ones I created in Outlook online. Now they're the only ones in my Apple Mail on my Mini. So, my question is: how do I move my old Apple Mail mailboxes from the old iMac and insert them into my Apple Mail on the new Mini? Any help appreciated.