The basic visible design is usually
- two Y walls, North and South that carry the baulk of the heat generating equipment, payload or platform, though the latter is usually concentrated at the -Z end.
- at least two floors, one near the -Z end one near the +Z (Earth pointing) end. There could be additional floors at the +Z end for payload equipment that needs a proximity to the Earth face. The platform equipment is grouped around the -Z floor.
- the +-X (East/West) sides are less uniform and could be panels or left with no structural element.
- regardless of the structural elements the +-Z and +-X sides would be blanketed except for where equipment protrudes
Internally the basic load bearing structure is usually a combination of a central thrust cylinder and shear panels. The central cylinder connects the floors. There is probably an example somewhere that doesn't have a central cylinder but it is typical that there is one. The shear panels are interior vertical panels that connect the central cylinder, floors and Y walls.
Propellant tanks are usually arrange either:
- stacked inside the central cylinder
- four, with one in each quadrant around a small central cylinder
- a combination of the above
Interior structure is usually painted black, as are the bulk of internally mounted equipments. Propellant tanks are usually blanketed.
Doing a search for "satellite name" + "AIT" or "clean room" might show up some other part assembled photos. The first image is a screenshot from about 51 seconds into one of the videos on this link of Thor 7. The outside of a Y wall is visible at the left of the image with antenna feed horns visible protruding from under blankets at the top (+Z) end. On of the -+X walls is open on the right of the image and one can vaguely see that its all black in side but the central cylinder and Y walls can be seen at least.
The grey items protruding from the edges of the Y walls are radiation cooled TWTAs and it is probable the business-end of these devices that can be seen in rows on the inside of the far Y wall.
The image below is the first drawing I could find. The sharper eyed amongst you will notice that its not a geostationary communications satellite but regardless it does serve to illustrate the concepts above.
- The floors, central cylinder and shear walls are all visible.
- The solar panels are shown folded flat against the Y walls and aren't that easy to distinguish and this design has panels on the +-X walls (which would be East West on a geostationary satellite).
- Note in this case the apogee kick motor is shown as a solid motor, this is universally not the case these days and would either be a bipropellant engine in the same place or some external arrangement of electric thrusters.
Finally, to answer the sub-questions:
I think "doors" are rare. It is more likely that there would be access gaps in panels that are covered in blankets for flight.
There would be next to no pressurised items except perhaps for reaction/momentum wheels, unless you are talking about one of the older Russian designs where the whole interior compartment is pressurised.