How dose fwb happen/start?

It may be because I (M20) am a virgin and kinda socially inept (mainly make friends through other friends) but I just don't understand, how dose fwb even start?

I know situations are different but, do friends just start flirting with eachother and then asking? Or is it like one of them think the other is flirting and ask if they want to Netflix and chill?

How dose it start or how do they know if they'd even be open to it?