I like to farm machines in the one safehouse in the middle of the farmlands (the shelter left of the Bridge called Boo Bruk AB). and sometimes there is a giant royal rumble of soviet anf Fnix machines and sometimes this Area is completaly empty.
So how does the system work? Can i trigger the spawn? Or do i have to switch regions often for them to respawn?
After you fight in the area, leave it for an hour or more. When you fast travel there, look at the loot boxes. If you can loot them again, the area has reset. That is how it has worked the last 4 or more years so expect it to change if there are more updates.
is it 1 ingame Hour or one hour of playtime?
Thanks for your help 
In game. The trick is to visit the area often and see what has been reset on the map, so you get a sense of how much time it takes for EVERYTHING to be reset.
After you finish a battle, leave and come right back. You may notice the destroyed machines are gone. Leave and come back, you may notice a few loot boxes are reset but others are not and there may be a few of the machines back, but not all. Given enough time everything is reset.
You will also notice that it is not always the same machines that are in a given area. Or, you don’t visit soon enough after the area resets and there are more machines than normal. Given enough time, you may find an active area has twice as many machines.