I’d recommend trying to do it again using the description by MichMich
https://github.com/MichMich/MagicMirror/wiki/Auto-Starting-MagicMirror
It is crystal clear from what I see, also describes what to do.
"the doc for ‘pm2 startup’ doesn’t help me…
‘will give you the command u need to execute’
execute when, how? if unattended, who is ‘you’?"
No offense, but btoh the doc and your terminal will show you exactly what to do. Copy a line of text the terminal throws out and execute it again.
That’s the reason I’d suggest to do it again with a bit more patience. :-)