How can I make friends as an adult?

Even though it can seem a little scary I would see if there are any local groups that do something you like. My parents were in a similar situation in their 40s. My dad went to a motorcycle club meeting and met some people. They asked him to go on a big charity ride. One thing led to another and today (10+ years later) they have a really awesome friend group they've known for years. It can be nerve wracking to put yourself out there. My biggest pieces of advice would be to not try and force anything and dont just assume the first group is going to work out. It might take time and you have to find more than one group (my dad tried 2 other motorcycle groups first and didn't like them much)