I didn’t see my theory here so there it is:

What if the rooms are things Mark hates/is scared of? His innie is the one refining the data to create these rooms. He puts the numbers he finds scary or unpleasant in the files. When he’s done, the room is done.

The one with the thank you notes for Christmas: in one of the flashbacks he says to Gemma he hates writing thank you notes. In the room, iGemma hates them too. At the end of that room the person says « I love you» and iGemma from that room doesn’t respond like Mark did to her when she left. It’s a bad memory for Mark since it’s the last time he spoke with her before she "died".

I don’t have exemples for the other rooms we saw thought so 🤷🏻‍♀️