It can be a lot more difficult to develop and maintain good friendships as an adult. You really need a longstanding topic to bond over. When you are younger you have education and daily interaction to bring you together.
I’ve found that to build friendships you really need to do something, enjoy something together and do it with some regularity. This is why joining a group where there is already a joint focus is recommended.
It is a bit of putting yourself out there and it can feel really lonely, I know.