https://archive.org/ has old media of all sorts including game magazines, demos trailers and manuals.
a lot of video material has been uploaded to youtube, that was formerly hosted elsewhere. hell i've watched the old eoLithic frag movie from like 2002 a week ago.
vimm's lair is also worth a visit if your are looking for old game manuals.
Yeah I've sometimes used all those sites at some point, but they're so vast (or have only manuals, like vimm's lair). I was interested in finding something a bit more curated and game-oriented.
Not the most searchable since it's main use is through it's api (I use it through retrodeck/ES-DE which is for ROM libraries) but screenscraper.fr contains scans of boxart, title art, videos from the games, manuals that used to come with physical copies and a lot more.
If you want retro ROMs, I can't help you since I don't know if I can link to those types of sites but ask on !piracy@lemmy.dbzer0.com and check their wiki linked at the top of the community.