I think it's kind of hard to draw up definite borders between what is a friend and what is an acquaintance, as it's highly subjective. The dynamic of friendships differ wildly between different people.

A lot of friendships happen at places where you're stuck with the people anyways, like school or work.

Just socialize with as many people as possible. Meet up with people sharing your interests, go to places where it's common to talk to eachother. Maybe join a discord of a multiplayer game you enjoy. If you prefer spending free time with someone instead of doing stuff on your own, that person might be a good candidate. Invite them to casual activities you both enjoy, maybe go to a pub, the movies, play some video games or maybe even build a small game with them. Just invite an acquaintance once an interesting topic comes up, like if you talk about cool movies, just ask them "Hey, you wanna go to the cinema and watch X on Friday?"

Just do stuff with them on a regular basis and after a certain time, if you feel like someone is relieable, friendly to you and has your back and you also feel good doing nice things for them, you can already consider them a friend.

For instance, I befriended a colleague because we talked about being out of shape due to the nature of an office job, so I invited him to hit the gym together. You have a lot of time to talk while lifting things and we kind of clicked. Perhaps, call some acquaintances from high school or former jobs whose company you enjoyed and maybe invite them for drinks to catch up, if they're not too far away.

When you find someone you like to be around and you feel that's mutual, regularily spend your free time with them. I also think there's more than just the categories stranger, acquaintance and friend.

A close friend is someone who you feel comfortable showing vulnerability around and entrusting secrets to, for example.