This doesn't help if you're trying to bootstrap from nothing.
> just join a sports league
I spent solid chunks of childhood being forced into sports programs when I at best had below-average physical abilities. I even tried one summer as an adult to play in a league and I discovered that the physical gap had just widened and widened and it just helped isolate me until I quit going.
It's really great that people find things like sports leagues or meetups useful for this, but none of it overlaps with my mostly solo activities.
Unfortunately, that's how it is if you don't share interest with the extroverted crowds: you're just stuck looking for scraps.