So how is this Facility serviced ? When we are present, we see nobody, NPCs or otherwise ?
It isn't, not really. The game just calculates such things in the background and sets the values from that. This is called the BGS (background simulator). The entire economy is simulated, not real. That's why you get some very odd happenings, like prices being high with zero demand. Like I said, it's smoke and mirrors to make the galaxy appear to be living.
After all, they can't very well have millions and millions of NPCs going at once, your computer would not be able to handle it. So there's a small 'instance' around where you are, and NPCs, stations, planets, etc. are spawned into it when you arrive, so that it looks to you like they were always there.
You are peeking behind the curtain, so to speak, with this question.