I’m not sure what defines FB/FWB/relationship. I think the boundaries are quite blurred and depend on the people involved and what you’ve agreed.
For me, FB is literally just sex. A booty call, prearranged shags, no real communication beyond texts or calls arranging to get together for sex.
For me, FWB is sex with some dating and non sex time together. Meals, drinks, hanging out. But no real integration of each other’s lives. Not meeting friends or family, not being a part of each other’s lives in any menaingful way.
A relationship to me is FWB with the extra stuff I’ve mentioned above. Looking longer term and the bigger picture. Not necessarily moving in together or marriage and kids, but overlapping lives more.
I’m not sure I’m doing it right though as my FWB is so lovely I think I want more, but he doesn’t so I just have to relax and enjoy it for what it is. And it is lovely, but as soon as I get to the point where’s it’s NOT enough I need to switch it off so I don’t get too hurt.