How does the boyfriend/girlfriend thing start ?

I'm a 22 M here.

I never had any relationships or anything like that and I was just wondering, how and when does it happen ?

Like let's say you meet someone you like. You talk to them and get to know them, then ask them out. This part I understand.

It's the part in between friendship/dating and relationship that I don't get. Do you just have sex with them and then say let's get together ? Or you just tell them one day, just like that, when you decide to ?

What's the transition like, how does it happen and when ?

Also, when dating, do you refer to this person as "person I'm dating", "my friend", or "boy/girlfriend" ?

Thanks !