Thanks! My next life-affirming project is a netsearch for GRUB stuff.
So far I have
https://www.gnu.org/software/grub/manual/grub/grub.html
https://linuxhint.com/grub_boot_loader_full_tutorial/
I borrowed Linux in a Nutshell from a library.
Chapter 3 Linux Commands, 471 pages, half the book! -...
$51 944 pages, Linux in a Nutshell - published 13 years ago
$31 464 pages, How Linux Works - Published last year
$53 830 pages, Linux: The Complete Reference - Published 2007
My first step is setting GRUB multi-boot, with Ubuntu on one SSD and Windows on another SSD. Which book covers that?