Why does systemd kill processes before ExecStop is processed?

Question

linux

In order not to be verbose, I will give a link to my own question on SO.
In a nutshell:
There is a service. Spawns many processes.
On reboot, during the execution of ExecStop, all the processes that I have to terminate myself are already killed.
At what point do processes send signals, and can this be avoided?
For example:

1. specifying the Before=smth parameter in the [Unit] section
   
2. indicate that the processes spawned by the service belong to this service and should not be killed just like that, for no reason
   
3. ???