@JdeBP @glitzersachen it's worth volunteering that boot(8) is one of several man pages I was scrutinizing to try to figure out how any of this worked, and it did help me eventually figure it out, but slower than it probably should have. There's several bits of NetBSD terminology (like primary vs secondary boot programs) that weren't obvious in how they mapped to terms like EFI.
Would reading some proposed #FreeBSD doco help?
* http://jdebp.info/Proposals/freebsd-boot-manual-page.html
I wrote that for someone who observed that man boot was xyr goto, and it didn't exist on non-x86 architectures in FreeBSD.
I also wrote it up in a different way. The boot(8) manual for non-x86 was written to be an overview and pointers to other manual pages. The FGA is a slightly different approach.
http://jdebp.info/FGA/freebsd-boot-process.html
There are similar ideas on #NetBSD , although the names of things aren't the same. The FGA touches on the multiple stages thing. The reality of course is tha most of that goes away with #EFI.