Nexus mods doesn't require owning a game since it's a third party site. Even steam workshop stuff can usually be gotten in one way or another too. I'm not sure what the go-to mods are these days; I used project nevada back in the day but that's apparently deprecated and a broken mess now, while its main features are done better and more stably by other more tightly focused mods now. Someone linked me a big list breaking all the elements down and saying what the replacement is, but I forgot to save the link and can't find it in my replies now.