Very good question!
In Space there is no air to put thrust on any other thing to expect any action-reaction forces.
You are right, there is not much to push on. In low Earth orbit there is a very tiny amount of air left. It's not very useful to use to maneuver much, but it's enough to gradually lower the orbits of anything up there.
So orbiting spacecraft ...
Partial answer to
What if the LOX/LH2 tanks were located on the Orbiter instead of as a separate module?
The original concept of the fully reusable shuttle system included an Orbiter with internal tankage such as you suggest.
However, there was also a separate, much larger, reusable Booster (also with internal tankage) which carried the majority of the ...