Not sure if when the knowledge about it's design was made public, but name Losharik gave everyone a hint. It is a cartoon character that's made from spheres/balls connected with a wire. This is a logical way to create a submarine that's intended to go very deep, spheres are the best shape to handle high pressures. So logical conclusion based on the name was that internally it's based around connected spheres.

Submarine itself looks cigar shape, but that is just her outer shell, it's pretty certain that outer shell is not pressure sealed but rather that internal spheres withstand the pressure while the just the shell around it.