In other words, which file causes Siri to stop changing PID?
Mac Preferences -> Siri -> uncheck Enable Siri checkbox. -> Reboot.
This will stop Siri from running.
To answer your question about changing PIDs, PID is a Process ID. Anytime a new process is created, it'll always change its ID to a new one. There is no changing this as it's a flow of how processes and multithreading work.