in the past, on my old PC, I saved a bunch of songs into iTunes either by downloading from a cd or by converting a YouTube file into mp3 form.
Those songs are only on that PC. They are not "in iTunes"
Therefore I didn’t need to have a subscription to Apple Music.
You do if you wish to access that music from another device. Or a paid subscription to iTunes Match.
This pc is brand new and I did update iTunes so I don’t know why I can’t see all of my music.
It is music that I saved into my iTunes so I should be able to see it when I open iTunes on any pc and log in. But for some weird reason I’m only seeing things that I have purchased, not anything that I downloaded myself.
I have explained this already. That "old" music you got from somewhere other than iTunes purchases, lives only on that PC. If you migrate/move the library to your new PC, it will be there.
How to move your iTunes library to a new computer - Apple Support