ENTIRE POST CONTAINS SPOILERS
To "lightly summarise" for OP. @EdmundSlackbladder as per request
When Hap threw OA out of his car he stole her movement and travelled with Homer, Rachel, Scott And Renata to another dimension were they all assumed the identities of other versions of theirselves in a PU, and were aware they'd done this, except for Homer who remain his PU self and was not overwritten by Original Homer.
For Renata, Scott and Rachel this was merely swapping one prison for another as they became psych patients instead of hostages. Hap found that he had become a preeminent and celebrated psychiatrist in this life. Hap researched into OA in this reality found Nina and found her living a very different life.
Nina was engaged to Pierre Ruskin, exceptionally wealthy and culturally Russian, never had an NDE etc.
New Hap Met them socially after real Nina read his book, he inveigled his way into their lives, planted the puzzle app, that harvests kids into Ruskins subconscious during therapy. He made sure that he was the first person that Ruskin would turn to if "Nina became psychotic" aka got overwritten by Prairie.
Even though Nina is not OA she is considered supernaturally gifted in the Russian community (psychic octopus, worst bit in the show, very jump the shark) and obviously has a regular "bit" at the club.
Prairie/OA transferred into Nina on the Ferry, at the point she was shot in Season 1, and overwrote Nina's subconscious.
Nina's bloke owns the app company and they record people's dreams with Silent Witness lady. The house is built on a Native American sweat lodge type place. Ruskin/Hap is using the kids to solve the puzzle which involves finding "the Rose Window" - kids are dying due to mercury poisoning.
All the young people from S1 have been sucked in by the puzzle in the parallel universe due to meeting OA in their primary universe.
FBI man is some kind of angel. French woman who is a traveller got on to Hap and legged it.
Homer has become too suspicious of Hap so Hap offers to release Scott, but not Renata, Scott says he will help him one last time if he will also free Renata and they try and access the house.
French woman gets OA to merge with Nina.
By doing this they manage to get Homer to merge with his two selves.
Rachel said it's only safe for BBA to travel because the others are all trapped but somehow Steve and Buck have managed it.
Karim gets to the Rose Window, he unifies Buck and Michelle after seeing Hap and Nina/OA jump to a world were they are both successful actors making a film about this exact premise.
JASON ISAACS PLAYING HAP IS NOW HAP PLAYING JASON ISAACS 🔥😂🤯
Clearly the OA, Hap, Steve And Buck have all made it to that universe, others unknown.
Fucking class!