I downloaded the movie after seeing it in theatre to once again enjoy it from the comfort of my home. Seeing 2160p, I thought it's going to be a webcam rip but the title says webrip. Where is this leaked from that has Dolby Vision on a movie still in theatre?
Would that be like on a tripod in the audience area recording the screen?
yes.
Seems lossy somehow
it is! but that's where Shannon sampling theorem comes in. The sampling only needs to be twice as good as the source, and then you can reconstruct the source perfectly. (with some assumptions, e.g. correct color gamut, focal point, etc.).