Espionage is used to find out things and steal tech from other races/nations/whatever.
Xenology is the skill required to excavate ruins.
When you put a team on the orbital body in question, it will start working.
After some time, which might or might not take several months, they will tell you about their findings, aka, how many installations are recoverable.
Then you have to send Engineer Brigades to the Planet, preferably lead by Officers with a Xenology Bonus, and they will start recovering those installations.
Most successfully recovered Installations have a small chance to yield a tech.