The film is run through the camera & projector sideways, allowing for a larger frame to be laid down on the same stock.

https://secureservercdn.net/198.71.233.183/gkz.aeb.myftpuplo...

Yes, it’s an even larger format than traditional 70mm. The cells are physically larger. Anecdote: Nolan worked with Kodak to invent black & white IMAX film for Oppenheimer.

https://www.kodak.com/en/motion/blog-post/oppenheimer/