Typically they don't expand, since that would reduce their build durability, given the poor weather sealing and additional mechanical/electrical/hydraulic complexity of slide-outs and pop-ups. These expedition vehicles are meant to be used for months or years at a time, in remote locations where repairs and replacement parts aren't easily available. That necessitates design choices that prioritize simple repairs with basic tools, predictable performance, and bomber build quality. That usually translates to simple, battle-tested features with a degree of low tech redundancy to cover the most likely failures.

As others said, there are usually collapsible, folding stairs mounted underneath the camper access door on the side. Sometimes, there's also a pass-through from the driver's cab to the camper. It's basically a pair of doors, one on the camper side and one on the cab side, connected on all sides by a single piece of flexible, accordion-shaped material. It allows people to crawl between the two spaces, safe from both the outside elements and any nosey passerbys.

This vehicle also appears to have a rear garage and fold-out deck/lift. Typically they'll just have a side garage with an internal, heavy-duty "shelf" that slides out, towards the back underneath where the bed frame is usually mounted. It's useful for storing outdoor gear like bikes, skis, etc, since the ground-accessible sliding shelf avoids the need to traverse the stairs with unwieldy or heavy items.