It's mainly from the Doppler Effect. Article explaining it: https://io9.gizmodo.com/the-truth-behind-interstellars-scientifically-accurate-1686120318
I'd also recommend Kip Thorne's The Science of Interstellar. He gets into the details a lot deeper.
Reading thru the comments, I see a few people slamming Interstellar for its creative license towards the end and for its use of typical movie-science tropes like “solving the gravity equation”. In my view, those are legitimate beefs to an extent but they also apply to every other sci-fi movie ever made.
If one is interested in judging Interstellar solely on the basis of the physics that it portrays and on how accurately it does so, then I suggest reading this book:
Science of Interstellar
Edit (a day later): For a more technical - and also more critical - analysis of the physics in Interstellar, one might be interested in reading this paper, available as a preprint from arXiv:
The Warped Science of Interstellar
Should be a household name. He has written several books and was consulted by Christopher Nolan for the movie Interstellar:
Kip Thorne wrote a book on it:
More is accurate in that movie than first appears.
He even authored a book called
The Science of Interstellar
But not the parts you think:
Kip Thorne discusses where the physics is wrong. He also discusses why it is wrong for the sake of the movie, but most of it is right.
A lot of people think certain parts are wrong in the movie that are actually accurate.