You need to have a vast amount of knowledge of the systems installed in a building. It is far more than just having a couple plumbers and electricians running around fixing things.
I got my start as a field service technician by working on UPS and battery systems. I expanded my knowledge into switchgear and HVAC units.
After doing that for almost eight years (after six years of being an electronics technician in the Navy), I got into facility management. At that point, I started learning about generators, fire detection and suppression system, then finally construction and project management.