I started climbing and gained some friends through that. Activities with other peoples have helped me make friends. Then you invite them to spend time with you to do different activities, they invite you to do other things. And you make a little web of friendships.

Social activities of any kinds are where it's at, and many others who've commented here have the right idea.

I'm dealing with this myself, and as others have said as people exit education and enter the workforce, there's often fewer opportunities to meet new people and develop friendships. In school you had multiple classes with all new people to get to know, in the workplace you're usually working with the same dozen or so people. I personally also have a detached workplace persona that's very different from me outside of work, and that barrier naturally makes it harder to make genuine connections.

Local events like farmers markets or board game nights at a local game store are great ways to meet new people, it just takes some proactive effort to find them, actually show up, and be open to conversation.

Most friendships are based around convince. After a long enough period, and a little intentionality, you drift into friendships that based simply on the fact that you're friends.

Take a moment and think about where you've met your friends over your life. School, work, sports, volunteering, your neighborhood, etc. These are all excuses to get together and see each other over and over that aren't focused on the relationship itself. After a little while, you figure out which people in the group you like that have the time and interest to be friends with you. You won't always get your way, but you can literally put yourself out there as much as possible and make friends when the opportunities arise.

You don't have to be the life of the party, you just need a reasonable excuse to see the same people over and over.

I'm very extroverted. I have no trouble making new friends that are interested in a hobby or like to meet up at a certain place. In the 24 years since I graduated high school haven't met a new friend who just wants to meet up, hang out, and do whatever because life is more fun with friends. Back in my school days I was pretty bad at making new friends but the friends I did make were legit. I still keep in touch with them and we don't need a plan to have fun.

If anyone has any insights or tips on how to make solid friends as an adult I'd love to hear it.

I think it's more of a structural issue. During adolescence we're incentivized to socialize with others. School is a community environment, with one of its primary functions being to socialize people gradually into adulthood. Once out of this environment, building connections with others takes active effort. No longer are we encouraged to make friends by our surroundings.

A lot of great places to meet new people and start to form bonds with them are community or interest groups. If you have a hobby or want to get into one, there are generally plenty of options in your local area. Sites like Meetup can be a great starting place for finding these kinds of groups

It helps to get a hobby that involves other people. I've met lots of friendly people playing board games, and the hobby gives you something natural to talk about.

It is a little funny though. A lot of the time people give that as advice to meet women. I'm a man, and when I got into a hobby that interested me, it was 90% men that I met! It's anecdotal, sure, but many of the women I have met were coming with their SOs. Still, it's been fun, but I wonder what hobby I need to get into to meet all the women.

I may finally break down and get an account on Bumble or something. My work is male-dominated for various reasons I have little influence over, I don't want to start going to church (though I could probably tolerate the local Unitarian Universalist congregation), and I don't like to drink either, so bars are out. I'm exjw with JW family, so family matchmaking is out. I might not have many other options, though honestly I don't feel like I need to be in a relationship either, so I suppose I can wait.

I moved to another country after graduating college and been staying for more than 5 years. I made 2 good friends, after all this time. 1 of them got married and having kids, we lost touch quickly after. And the other one is still going on only for one activity, hiking, which we only meet up a few times a year. I learned that I gotta put the energy to invite them sometimes.