how to import FROM Thunderbird and Betterbird

I have a bunch of mailaccounts with different domains on Thunderbird that I want to migrate here - ie away from Thunderbird.. I can make a zip in Thunderbird with the lot but I can find nowhere to import neither as zip nor anything else?

Also I have even more on Betterbird - and old folders - that I want to migrate to have all in one (European) place, but can find nothing about how. Again from Betterbird to here.

I use POP for both so the stuff is on my machine and not online anywhere. (There is absolutely no way I'll use grand thief Google to move anything, even if I understood the explanation).

I want out of Thunderbird as I do not consider it safe. I changed pc from win to linux, handclearing it first, and somehow T knew my accounts and mails and imported it as soon as I opened Thunderbird. No way possible to have found it on machine as it had been reset byte by byte, so it must have been copied and saved elsewhere and not by me nor with my consent. So I want out.

when I migrated from outloook there was a special program for it. seems to be here as well only not mentioning Thunderbird nor Betterbird as something you want to leave.

I have the domains, accounts etc on my webhotel with my webpages, but want something like this (or rahter like the old outlook) for mails.