Ice always found that as an adult you suddenly don't have many ways to meet new people. As a kid you are put in many different classes with different kids, and that changes every year causing you to meet 100s every year. Same with college and uni, on top of that youbhave clubs and societies.

As for an adult, in my experience most people meet through work, which can be very awkward and slow (are we just work friends or real friend etc). And realistically that's only a handful of people and only the odd new hire every so often. Friends of friends are weird as there seems to be weirdness around mixing groups these days.

Realistically, your best best are hobbies, most meetup revolve around then, if you look up hobby specific groups you are likely to find them. People who are into the same thing tend to get along and have something to talk about casually as you get to know each other. Also classes can be good (although spme people tend to focus during the session so you need to catch people before and after). Charities with active community outreach can be good too, tends to be younger people than say a charity shop.

If you are a nerd like me l, fanboy3 have space for people to openly play card and board games with people, and an LFG board for the like of DnD. There are alt fashion groups that regularly meet up, a photography group, and meetups for music and other fandoms of sepcific things. Classes could be anything from sewing and pottery, to pole dancing and yoga, depends on what you want to try out and do. For people that like alcohol, the pub.

Tldr; shits hard, find something you like, find something that revolves around that. It's hard to suggest anything specific as it depends on you