Heh coming from the UK I didn't pick up half of the references at the time either! But with characters like Warren, Xander, and Willow I can hand-wave it as them being likely to have watched a lot of old and geekier shows maybe? They're quoting old sci fi movies to one another in When She Was Bad, they're watching bad tv in Reptile Boy when they can't afford to go out, so I always imagine them watching a ton of old tv shows and movies, particularly when they were younger and before Buffy came to town

Cordelia bringing up the Zeppo is always a reference I struggle with more, some of the references she made seemed a bit more unlikely for her character