Changing the default scheduler

Hello, I am a new developer trying to learn how to tinker with the kernel. I searched on the internet but couldn't find much info about this (and couldn't find any info up to date). My question is, how does the kernel decide which is the default scheduler that all processes start with? I can change the scheduler of a process by sched_setscheduler(), but how do I change *all* processes from using the CFS scheduler to, for example, the RR scheduler? Sorry if this is too basic, but I don't know where to search for this. If you guys could point me places where I can learn more about this, I would be grateful.