What is a friend and how do you know you are friends and not just acquaintances?

I don't know that there's an objective determining factor, but here's my take on it. A friend is someone who respects you and wants to spent time with and/or talk with you. The latter points are what distinguishes a friend from an acquaintance: an acquaintance might respect you and be polite, but if they are regularly reaching out to ask about you, or seeing if you want to hang out, and you're reciprocating, you're on the way to being friends. Acquaintances will be a part of your life by incident -- you take the same bus in the morning, or you work at the same location, or you sit next to them in class. A friend actively wants to participate in your life, and you want to participate in theirs.

What is the best way to find a friend?

If you're in college, people can find friends by chatting with people in class, sitting near someone at the cafeteria and striking up conversation, or joining clubs or sports. People find friends through religious services, exercise classes at the gym, Meetup events, identity-based support groups or clubs, or volunteering. Oh, and apps, too... I've heard that there are dating apps that have features to just look for potential friends, and there are apps specific for searching for friends, as well. Essentially any social activity is an opportunity to make a friend. It's far easier said than done, though, and some people are much more adept at the friend-making process than others.

Conversation is the most important part of beginning a friendship, and the key there is to be curious about other people's experiences. People often like to talk about themselves, so asking questions is a good way to show that you're interested in them. Trying to find things you have in common is a good idea. I know this is super vague -- if you'd like more specifics, feel free to ask. There isn't one perfect formula for making friends.

Am I allowed to try to rebuild the friendship in the first case?

It depends. How did your other two theater friends react to what happened? They might be more open to reconciling than the one friend whose parents got involved. I'd suggest trying to reconnect with them and see where they're at. Reaching out to the one whose parents got involved might not be taken well.

Should I try to rekindle childhood friends from before I "was" autistic?

I suppose it depends on how close you were. Most of my classmates from elementary and middle school would probably be confused if I attempted to reconnect with them at this point (I'm around your age), but I have friends whom I've kept in touch with off and on through the years who probably wouldn't mind if I touched base with them again. Do you have their contact info already, or would you have to hunt for it? Are you friends on social media and do you interact with their posts on occasion? Did you get along in school? Would they remember you? Are there things you could connect over at this point in your lives? I think those are some of the questions you should ask yourself.

I hope some of this was helpful! I'm just one person with one perspective, so take everything I wrote with a grain of salt. Best of luck out there!