How did yall get into fwb relationship/hookups?

I know I'm gonna sound stupid but although I understand how dating hookups work I don't understand how hooking up with a friend works

This is gonna sound stupid but I (M21) don't understand how people do it, like literally almost all my friends do hookups with thier friends (both my male and female friends) and I don't understand how people get into those situations

Like how do you just go from friends hanging out and talking to next second your fuckin eachother especially since I hear some people say don't talk about it before hand and it just happens or some say you need to talk about it or it's manipulation