It's very difficult and mostly luck.
Friends couldn't introduce me to anyone they knew, because the single men in their circle were usually single for good reason and completely unsuitable/ had major issues.
Once you're past your 20s you can't get involved in workplace relationships, it's completely unprofessional (a blind eye gets turned when you're younger/ in a junior role but there's definitely a cut off). Plus lots of women work in female heavy environments so no opportunities there.
Hobbies...a friend of mine went to dance classes. It was all couples, ditto local 'social' groups. She ended up feeling ostracised as the only singleton. I used to go to parkruns to try and make new friends (was in a relationship by then), went every week for 4 months and never managed to get beyond a hello, most people just turned their backs and chatted in their established groups. And that was just to try and find friends!
I met my (ex) partner online almost a decade ago, and that was complete chance, after years of experiences that were anything from disappointing to upsetting and dangerous. My MH and self esteem were permanently damaged by all the gameplaying, ghosting, lying and duplicity. It's such a toxic setup.
I split with ex recently and on view of the above expect I'll now live the rest of my life single. I think once you've missed the early ops of uni/ initial work, it's very difficult to meet anyone worthwhile.
Never were the lyrics to Eleanor Rigby so apposite... 'all the lonely people...'