New Mac won't boot

Had previous Mac with Monterey. Cloned to an external disk (SSD). Bought "new" second Mac which booted just fine in Macos 14.4 something (Sonora?). Then used this merge/transfer option to bring user account, apps and whatnot to "new" Mac. Seemed to work fine, took some 40 or so minutes but no errors or warnings.

But now, "new" mac with Macos 14.4 won't boot. Regardless what I try, it seemingly starts, but eventually reboots itself. If I leave it alone, it just keeps on rebooting itself.

It doesn't even boot from the external disk with Monterey (asks for some password I haven't heard of but I'm obviously supposed to get from Apple in some mysteriously way and that might take days or more)

When trying Cmnd R, it asks e.g. for reinstalling Monterey. But I don't have Monterey on the startup disk. It has 14.4 (Sonora or whatever the same is. On a side note: been using Mac since stone age, version 7.9 or something like that. I truly hate this idiocy they began when they started using Linux and renaming OS with ridiculous names of felines and then mountains instead of just numbers. Sorry, had to vent)

Kindly asking you to bear with my rant. But I'm baffled and really ****** off.

Can anyone help? Here or dan at proformica dot com.