As a shut in with social anxiety and very introverted I’m wondering how to find friends now that I got better.

Like others have rightly suggested: hobbies, volunteering, taking classes. Imho, hobbies with IRL encounters are great. You can also check moist local public libraries, they will often have activities.

Are you into sketching or anything art-related (photo, painting,...)? Try to find groups, classes,... Urban sketching groups should be the simpler to find in many cities, no matter how big or small they are. People going out together to make some (street) photography, maybe?

Do you play chess? There are cafés, clubs, shops that will organize encounters or simply let anyone play some games with other people. Here in Paris, beside a shop or maybe two where you're almost sure to meet other players (not to play there as there is no room for that, but to chat), the first place I would suggest any newcomer to go and have a look, weather allowing, is at the jardin du Luxemboug. There is a dedicated spot where anyone can meet other players and start playing. Chess sure is a great way to meet and start talking to people be it as a player or as a spectator.

Scale modelling is too. Be part of a club with regular meetings. Imho this is one of the best way for anyone looking to spend hours discussing with other people that often are more than willing to talk about a common interest: building/painting/criticizing scale models and, well, just having a chat about the hobby and whatever thematic a specific model kit may be about (race cars, tanks, planes, boats, scifi, figurines, such or such history period, it's endless). Scale modeling is not only a niche hobby it is also a rather lonely hobby too. So, any opportunity to meet a fellow modeler will often be appreciated — any impression I may have been a scale modeler myself would not be a pure coincidence, and even though I quit practicing the hobby I'm still always more than happy to talk about it.

very introverted

No mater what you decided to try, as an extreme introvert myself, I would say the most important thing to keep in mind is that there is no assurance this will work.

As a matter of fact, I would say that more often than not it won't work. And that's nothing to do with you (, or I, or anyone else)? It's just that most people are not going to those places/gatherings in order to be picked up as new friends by others. They go there to enjoy whatever it is they're enjoying doing.

Just be fine with it and don't ever think it is because of you that you did not met a new friend. Even if it happens you will screw something because you were too shy, or too anxious or whatever, that's fine. That's just how things are and how they work. This also why true friendship is at least as precious as love is, it's rare. Also, the older you get the harder it will be, says I a 50+ years old dude.

Just keep on trying... and don't push too hard or you risk pushing people away ;)