Except in the 90s, I could wield my OS of choice on the hardware I bought.

That's sadly true though. Also booting something was really simple.

Now we boot an embedded microcontroller (or CPU) which boots the main CPU which boots another OS semi-persistently to boot the main OS (if it's allowed).

Sometimes there are other processors needs to be up to allow processor to continue booting as well (these are mostly servers, but eh).