I believe it was a C-130 cargo plane, and that is how passengers have to sit. He’s fully strapped in so he’s in no danger of flying out the back, but I did wonder how they managed without oxygen because it looks like they were flying quite high. The thing on the back looks like a secondary ramp for personnel.
No O2 required below 10k feet
I’d guess they are about 5000 same as you would skydive from.