I think it's hard to make friends if you don't have a plan in any city. I recommend trying to join some group activities and seeing if you click with anyone there. Ex. Biking events, skating events, small concerts, art events, conventions, neighborhood, etc...

Most of my friends in NYC were extensions of friends I created elsewhere - so a friend of a friend or more. I think that's probably the easiest way though I was lucky.

Also surprisingly - reddit meetups for my neighborhood or part of the city were effective in me meeting some rad people who are friends till this day.

I probably wouldn't move back to DC, but it's for a variety of reasons you probably don't have. Growing up in the area meant I knew most of the venues and people with my hobbies. They keep things fresh, but I needed a personal change (maybe some parental distance) and NYC had a job that allowed me to try that new city. I think you appreciate a place more once you leave, and DC definitely has some great spaces and a lot to do - but it is different from NYC in many ways. Would it be a city I recommend trying? Always. I just wouldn't move back.