so when they meet for the first time in 27 yrs they're outside the chinese restaurant and Ben recognizes her right away and there's a flashback to the first time they met at school. during the dinner scene Bev mostly talks w Bill and Ben is clearly saddened by that but there's close to no chemistry bw Bill and Bev whatsoever.
later when they all go their separate ways in town to discover their old memories alone, Bev finds the haiku Ben had sent her in her old house but she doesn't remember it was written by him. meanwhile Ben goes back to school and has a vision kind of thing about him and Bev when they were young but its pennywise's tricks and he makes it look like Bev is disgusted by him but Ben won't believe it's real bc he knows Bev wouldn't act like that ever. after they get back to the motel they have a conversation about old times and Bev shows him the haiku and that it always meant so much to her, but she thinks that Bill wrote it and it's so heartbreaking Ben is just so sad in that moment but he doesn't tell her the truth.
when they go back in the sewers they're doing a ritual and everyone throws something in the fire that is their most meaningful memory from their time in derry, and Bev really hates the whole thing but she throws in Ben's poem even though she's really sad she won't have it anymore, and Ben throws in that page from his yearbook that Bev signed. he said he kept it in his wallet all these yrs bc he didn't want to forget her and never did and Bev starts to realize how he's felt abt her all along it's really bittersweet
when they're all trying to kill pennywise these two are usually side by side but one time pennywise separates them and tries to drown them both but Ben calls out to Bev and she hears it and gives her strength and they reach out to each other and save each other. they land on top of each other and it's when Bev realizes that Ben wrote her the haiku. they almost kiss but there's no time really cause their lives are at stake still.
after pennywise is finally killed the remaining losers go back to have a swim at the lake they used to jump into and Ben and Bev are very playful and they share a kiss underwater which is like the most beautiful thing ever!!! later there's a flash forward to all the losers living their lives a few weeks later and Ben and Bev are on a boat enjoying each other's company (and they also have a dog) so it's a very happy ending for them, perfect really