Anecdotal, but if you're looking for friends as an adult you should get a bicycle. I've moved 3 times in my adult life, and have made a solid group of friends each time by signing up for group rides, etc. Ages range from late twenties to seventies. You can usually find these on social media or by talking to a nearby bike shop.
This doesn't just apply to urban centers either. Gravel and mountain biking is huge, and generally the shops in your area will be catered to the most popular kind of biking.
As an adult (I'm 53) I've managed to make lasting friends in only two contexts:
1. My local arts community, where I've served as a board member and patron for many small organizations; and
2. Cycling. I'm a big roadie, but I dabble in gravel -- primarily for social reasons. This has actually been a larger source of friends than the theater stuff, but both have been big. The bonus with cycling is that you kill two birds with one stone: you exercise AND friends in the same activity!
Now, I'm sure that one could abstract these two things out to "volunteering" and "local sports," but I feel like cycling in particular is great for this.
> The bonus with cycling is that you kill two birds with one stone: you exercise AND friends in the same activity!
Make that three birds because you can travel to places!
One of my favorite things to do in the summer is ride with friends to the city center and grab some food, maybe a drink, and hang out before riding back. It's a leisurely ride on multi-use paths through parks, so even people that aren't comfortable or new to biking are willing to do it and you get to see cool parts of the city that aren't as accessible in a car, bus, or train.
This is very subjective, but cycling does not work for me in this context.
Obviously, everyone has their own goals when getting outside on a bike, from pure leasure/entertainment to hard fitness/training sessions. I'm somewhat in between closer to training camp.
Riding for 25 minutes casually, then getting a beer and talk for an hour, then riding another 20 minutes home is not for me. But riding with the training group is problematic too - your fitness level matters. If the group rides 26 kph average for 120 km but you only can give 23 - you are going to be left behind. Dropping and continuing solo or turning earlier - I was on the receiving end of that multiple times.
Eventually you realize most optimal trainings are done solo and they are kind of meditative and enjoyable when silent. And high exertion does not allow you to talk anyway.
OneWheel group rides, Bike group rides, Motorcycle group rides, plenty of opportunity. Mountain Biking is hit or miss, there are some snobs in that sport, road cycling as well, but for the most part all of these are great options to get outside and find like-minded people.
Maker groups, meet ups, etc for those not physically inclined. Book clubs, music clubs, and things exist. I met some really good friends from D&D groups and OneWheel rides.
The only reason I have friends as a mid 30's is because of mountain biking. At least where I am, the advocacy groups schedule lots of group activities - rides for all skill levels, trail building days, bi-annual meetups. And folks you meet when riding are usually friendly - I've met several riding buddies throughout the years just due to being on the same trails at the same time.
My hunch is that it has something to do with doing/accomplishing something "hard" together. I'm also guessing that the harder it is, the more bonding automatic bonding you get.
This doesn't just apply to urban centers either. Gravel and mountain biking is huge, and generally the shops in your area will be catered to the most popular kind of biking.