Description of classic and modern C64 graphic modes (studiostyle.sk)

The 8-Bit Guy has some great videos about how graphics worked on the older systems (including C=64): https://youtu.be/Tfh0ytz8S0k

If you're interested in coding something for C64, there are screencasts at http://64bites.com (by mehowte).

"Modern" meaning 1999, of course!

It looks like there's some more info and links at https://www.c64-wiki.com/index.php/Graphics_Modes