“Soooo, what exactly does it do for 780 days in space, hmmm?”
I would guess that the main purpose of the test, beyond the obvious checkouts of systems was to try to see if it could move around up there (change orbits) without being immediately detected. If it can, it could sneak up on enemy satellites and learn a lot about them.
That would be a big fat ‘no’. There is no stealth in space and the X-37B was tracked in real time for every second it was up in orbit - using commercial, private sector tech, no less. See for yourself:
Private space flight has made real time tracking of everything in orbit a necessity.
“Learn” by extracting them into the bay and re-entry. Pac Man of satellites and sat-weapons.