It seems that the primary way people make friends as adults is by befriending people they meet at group social situations: meetups, sporting leagues, parties, bars, etc.
But I have no idea how this works! Most of the time when I go to group social situations I either stand by myself awkwardly and don’t talk to anyone I don’t already know or I do talk to new people but then we’re barely acquaintances. The next time I go to the same group event, the cast of characters often shifts somewhat, and there’s no guarantee I get alone time with anybody I’ve met before. So I end up with vague acquaintances, and that’s it.
I strongly recommend getting involved in some activity where you will have an individual structurally-acknowledged socially-interactive role to play, so that your engagement with others doesn’t immediately take place on the plain of “hi I am talking because I am trying to make friends.”
Theater LARPing is super great for this. Normal theater is also pretty good if you can deal with the culture of normal theater.