yeah there is a setting for parked aircraft. I’m not at my PC now, so I don’t remember the exact setting, but should be easy to find in the traffic menu.
That is 100% random. The game will select random aircraft from its library (including community library if they’re enabled as AI traffic) and random liveries according to the density setting you use.
This is entirely separate from online live traffic which is mostly traffic in the air. It will park aircraft that arrive at an airport and those about to depart, but it does not control those already there when you load in. So the aprons will be 99% empty unless you add the setting for parked traffic above.
There is no way to control the amount of online live traffic. It’s controlled by what’s flying in the real world. The date is also irrelevant. It’ll always reproduce what is flying at the time you’re flying, or a few minutes earlier, to be precise. There a bit of lag.
The data used by the system is from https://flightaware.com/