From blessing Classic Mac OS, through SUM and the first Recovery partition in Mac OS X Lion, right up to 1 True Recovery on Apple silicon Macs.
SUM
From Startup Manager to Internet Recovery on Intel Macs, and how to engage each of those in an M1 Mac. With comprehensive diagrams.
Diagnostic, Recovery, Safe, Startup Manager, Verbose, reset SMC/NVRAM, firmware restore, Target Disk/Display, Single-User (SUM) in outline.
Single-user mode to run fsck to check and repair your boot disk became tricky with Fusion Drives. With a T2 chip, it’s not practical in its traditional form.
The Eclectic Light Company 