The way I see it is you either frequent a group or game enough to make familiar faces or you bring some existing friends to hang out with. I play terrors a lot and have been in kid-free social groups like Ancients for about a year and have made a few friends.

I feel like this applies to breaking into any social setting, but I think a big thing you can do is just be somewhere regularly with consistency. It only took a few weeks of joining the most populated terrors instance (which is almost always under the group "fusion pilot fan club") for the regulars to start recognizing me. Groups like Ancients are great kid-free spaces that aren't specific to any interest, but there's a lot of other interest-specific options that would speed up the process if you can find one that aligns with your interests, like clubbing groups, probably tons of board game groups, so many worlds have pool, darts chess, and various card prefabs so that's very popular - I'm 99% sure there's some chess meetups, movie watching groups (virtual theaters with spatial surround sound are really cool for this), lots of dancing groups because full body tracking can make that really fun, I'm sure anyone else could list more types of groups than me because I mostly play terrors and join ancients to be able to play a few games with full lobbies and no kids.