How to get rid of too many on my mac mailboxes

Dear Apple Friends,

I already read a couple of people the same issue as mine but I thought I would ask it again. I just recently upgraded to a new Mac studio four and I'm having all sorts of issues. This questions is about mail. After spending around four hours trying to figure out that you can have only have one SMTP account even though I have three email accounts, I'm trying to figure out a way to get rid of all these excess mailboxes. I thought by creating them in the favorite section near the top they would go away, but they just reappeared. I can't put them inside a folder. I can't really move them anywhere. I wish there was some way to group them to like all sent, all trash, and all junk, but I can't figure that out. I'm not sure anyone can help or if it's just another bad Apple design but anyway, any suggestions would be helpful. Thanks Rob