In all honesty, how does one make friends as an adult? I'm slowly starting to believe that it's impossible
I think as an adult, we're pickier about friends, because "liking the same stuff" is not enough for a meaningful relationship.
We want people who share:
Our energy (for me, it helps if the other person is autistic)
Try to find a group or activity where at least 2 of those are immediately present. For me, that would look like a queer meetup at a craft or game store, or an over-30 cosplay group. Online, I'd be drawn to a small historic fandom that leans older or has a lot of trans people in it.
Then, you start vibe checking folks, and perhaps exchange socials where you can learn more about a person in order to check off that third criteria from our list.
Not everyone is going to become besties, but you still could end up with various playmates that are fun to be around -- maybe you go hiking with Group A, or play D&D with Group B.
You can still be that kid on the playground who is like, "I think you're cool, wanna play?" Speed running friendships happens a lot in online and special event spaces. But it does take regular effort - you need to interact somehow, be that leaving comments on fanfics, participating in events, baking cookies for a meetup, etc. You have to put yourself out there.
Learning how to do small talk for the space you are in is very helpful, too. Remember to give the other person ample opportunities to talk.
Just like in romantic relationships, "nice" doesn't cut it as a personality trait -- you need to show what you bring to a friendship, be that as the supportive/encouraging type, the creative type, the organizing type, etc. So, spend some time figuring out what kind of friend you tend to be, or perhaps want to become.
Some overlapping advice posts:
Being autistic and how to look for help/community
I hope this helps. Personally, I've made much more meaningful friends as an adult -- most of my childhood friends turned into people I'd never want to meet as grownups. But I also had to step outside of my comfort zone in order to find people, which was a challenge as an autistic introvert.