[Part 1 of 2 because I Talk Too Much™ and exceeded the text limit for forum posts]

Alrighty, I waded through the thread a bit, but not all of it, sorry if I repeat stuff that's been said already lol. I just want to respond some before I forget what I want to say.

As someone who's very open about being autistic and hangs out a lot in the fringes of the online autistic community, I just figure I should weigh in.

To start, there's a saying that floats around in the autism community: "If you've met one autistic person, you've met one autistic person."
Keep in mind that autism is a spectrum; how different autistic people present, their preferences, their skills, and their deficits, can vary wildly. Men and women typically present with different traits as well, so my experiences will possibly read differently from the male autistics that weigh in. You'd need a lot of people to talk about their experiences to even come to a modicum of an understanding of it. The best way you can handle things in the future when you make a friend or just meet someone that discloses this, is to simply ask them about these things to learn their personal perspective and what they want or how they feel about things.

That said, on to the more specifics of your post. Some of what you said initially is a bit vague/broad, but I'll try to reply. So, an autistic person talking about how they feel they don't belong or don't fit in is certainly not super uncommon. The majority of us were "othered" for most of our lives. A lot of us can have trauma from being bullied. So even if we're in a group that's accepting of us, we can still sometimes feel as though we don't really belong there, that people don't like us, etc.
A lot of autistic people do something called "masking," which is essentially employing a set of copied or learned social skills to better communicate with non-autistics; this is a sort of survival skill that helps us avoid being ostracized or bullied. ((As a side note about that, doing this is VERY exhausting because a lot of effort and thought has to go into it, and learning how to do it doesn't make us any less autistic. This might be a kind of hit-or-miss metaphor, but think of it like running Window 10 in a VM on a Mac laptop with very limited battery life.))
In a sense, that feeling of not belonging could in some cases come down to the fact that they're having to try to act like "everyone else," because they're worried people won't understand them. This is just one idea of course, but it's one of many varied possibilities that wouldn't be innately apparent to you that could be contributing to this sort of issue for an autistic person.

On them not initiating conversation with you first, I'd bet money that sometimes it's that they feel as though they're going to annoy you somehow. It can take time for us to figure out that someone actually enjoys talking to us. Like...it can take a LONG TIME sometimes.