Since I first played MGS on the PSX I have been wondering about this.
To my understanding, MGS loads the geometry of the 'room' where you are and lets you move through it by using semi-fixed cameras that can follow you to a certain degree until your character gets in the range of another camera, and then seems to transition from the last angle of the previous camera to the initial angle of the current camera.
However, since this game does not seem to have a great deal of reverse engineering done, I wonder if anybody has replicated this scheme on any game (stealth or not) to study it.
As far as I know, the only one I have found with a similar approach is Resident Evil Code: Veronica, which switched from pre-rendered backgrounds to 3D but restricted the cameras available, as in the previous installments, giving the cameras certain degree of freedom in following the player (but less than MGS since RECV is about being afraid, not being stealthy).
I know I am asking a questio too broad and risk to be tagged as such... but it's been a lot of time since MGS got out and I still wonder about if anybody has more concrete answers to my guess about how MGS works.