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I didn't quite understand the ending:

1. All kids and BBA did those movements so that OA travels into another dimension or to save her from danger (in the final episode)?




2. What were all those bodies of kids doing there, was it from the future or from the dimension where she was Nina, how did they end up there (when Hap shows the research part to Nina)?




3. In the final scene, she falls on the ground and Steve greets Hap in the ambulance so the story will keep going (keeping the fact that Homer, Hap and OA will always be in one dimension simultaneously)?

Side note: I've taken to referring to each dimension by who OA is in that dimension. The original dimension is "Prairie Dimension" (PD), the second is "Nina Dimension" (ND) and the most recent one is "Brit Dimension" (BD), since OA appears to have jumped into a dimension where she's Brit Marling. This is just my personal system for keeping track of things.

All kids and teacher did those movements so that OA travels into another dimension or to save her from danger (in the final episode)?

It's unclear what, if anything, was accomplished by the kids in the "Prairie Dimension" doing the movements. I suspect it resulted in Steve jumping to the "Brit Dimension" along with OA et al, but that's just my assumption. We'll find out next season.

I think OA was already on her way there thanks to HAP's robots. HAP seemed to know what he was doing when he threatened to send OA to "a dimension where everyone calls you OA, but you yourself don't believe it."

The others back in PD were trying to "help OA" - which certainly would have been accomplished by sending Steve to join her. Others in that group may also have traveled - we haven't seen them in BD yet.

What were all those bodies of kids doing there was it like from the future or from the dimension where she was Nina, how they ended up there (when Hap shows the research part to Nina)?

As explained in the middle of season 2, people tend to "echo" and gravitate towards each other across multiple dimensions, and when traveling, people tend to jump into another dimensions where the people they most "resonated with" are still around them, in one capacity or another.

The kids in the pool were the ND versions of the gang from PD (Buck not included, obviously.) They were there because HAP found them, somehow. He probably didn't know their connection to OA at the time, but as above, people tend to gravitate to one another across various dimensions.

In the final scene its shown she falls on the ground and Steve greets Hap in the ambulance so it's like the story will keep going types (keeping the fact that Homer, Hap and OA will always stay in one dimension simultaneously)?

Homer, HAP and OA have been travelling together thus far, but that's not to say only those three are stuck together. Remember, everyone from HAP's dungeon initially jumped with him to ND. They might get separated (like Homer being "stuck" in the ceiling) or they might be joined by others (as above, I believe Steve at least made the jump from PD to BD.)

My personal working hypothesis is that everyone who's doing the movements at a single moment in time will jump (if they choose - there seems to be an element of choice involved.) Whether they all jump to the same place or not remains to be seen. For example, it seemed to me like Buck jumped into ND, since he alone was still alive there (I guess we don't know about BBA in ND, I don't think she was in the water.) Steve, on the other hand, appears to have gone straight across to BD with OA and HAP, perhaps since he was already dead in ND, or perhaps because he was choosing to join OA specifically.

The idea of Homer jumping into Steve's body (suggested in another answer) is certainly an interesting one, but I'm not sure that I buy it myself. It seems like people either jump into their own double (the lady HAP was with said she was always herself, just different versions of herself, IIRC, and she has traveled extensively) or they get stuck somewhere in between, like Homer did.

1. It was their intent to help the OA, but remains to be seen if they actually helped The OA/Prairie/Nina travel vs if any of them also traveled. Remember Dr. Hap had giant robot cubes that did the movements too.



2. 
   

   We learn from Dr. Hap experimenting that there are "seeds" in the brain and he believes he has found away to grow or germinate these seeds. The gardens that grow out of their ears (and into the water) are maps or "cosmic highways". He thinks he can now see where to travel to. So those bodies in the pool are [the counterparts of] people whose seeds he has grown into a cosmic water garden road map!

   
   
   

   
   

   The OA 2.08 Transcript:

   
   
   

   The house seems to trigger something in people. I don't know how it
   works, just that it does it. Something blooms inside their brain.
   Something dormant. A seed, maybe. What you see in there can be very
   challenging. Are you sure you're ready? Okay. Every human mind
   contains the multiverse. An actual garden of forking paths within us
   all. just waiting to be fertilized. "We shall not cease from
   exploration. The end of all exploring will be to arrive where we
   started and know the place for the first time."...These flowers
   offer a glimpse into other dimensions. I've made a crude map of the
   multiverse. Grown it, really. All we ever had to do to was to be
   able to imagine it to travel there. Like you did when you thought of
   Homer as you jumped. - Dr. Hap

   
   

   
   


3. 
   

   Things get meta! It's unclear if it is "our" Steve or not, but the dimension Hap and the OA travel to may be "a non-echo" dimension, because Dr. Hap is now an actor named "Jason Isaacs" and his wife was injured on set when she fell from a crane, while reenacting the last scene where Prairie and Hap were about to travel and Parie/The OA was rising up. However this Steve calls him "Dr. Hap", which may imply he did travel, but it could also be that this Steve is a "fan of the show" and only knows "Jason Isaacs" as his character Dr. Hap. It's unclear also if Homer traveled. Some people theorize that Homer is in Steve's body. Just to note: this new scenario matches Scott's NDE and ironically, the other traveler that Hap and Prairie met told Hap about one dimension where she herself was an actress...

   
   
   

   
   

   THE OA 2.07 Transcript:

   
   
   

   TRAVELER: So, the events in one dimension Affect the other dimensions around it. They echo. When people meet, and the story
   between them is strong, it echoes in nearby dimensions. Events will
   conspire to bring these people together.

   
   
   

   THE OA: So, when I met Buck in the abandoned house and started to tell him the story of my life in another dimension, Michelle begins
   playing the game that will lead her to Nina's house.

   
   
   

   THE TRAVELER: Mmm

   
   
   

   THE OA: Everything is connected.So, how do we escape this echo?

   
   
   

   TRAVELER: To leave an echo is very dangerous. You could find yourself inside a life completely unrecognizable to you. You would
   shatter yourself. Not to mention, you and Homer may not even know one
   another in a dimension outside an echo.

   
   

   
   

There's a lot in terms of the semantics that we still don't know. Like Hap didn't know about echo realities vs non-echo realities, where as The OA does. So that means that even if it was Hap's intent was to come to this specific reality where he and The OA are married and in love, he didn't know that there is a good chance that he will be lost to it and not have any control or memory from before.

We also don't know if Hap knew who Prairie's friends (Steve, BBA, Jesse, French, Buck) or rather their counterparts were, but it seems like he didn't, because he acts quite surprised when The OA begins to identify them, especially alternate Steve. And again we don't know if that knowledge is lost, since he isn't any kind of Dr. Hap/Percy/etc, but an actor Jason Isaacs...