You have to leverage the situation you're in, but something that works well for me is go climbing gym > compliment someone on their climb > strike conversation > if it goes well, ask for their insta (semi-public info) > slip into DMs (wanna go climbing again?) > make a move
Between each >, it's obviously only a step you can do if the vibes check, but it's one example of something that works. Also I won't lie doing this is a bit difficult, but you get better with practice. It also works great if you go to busy climbing gyms in different places at different times, since you'll always see new people. Final caveat is I do this for friends, since I'm already dating, but it works the same way to catch a bf/gf