Default tmpfs configuration

All;

I am a systems engineer that has a distracting curiosity. I was recently working on configuring a RHEL7 system for an Oracle database and following Oracle instructions on doing so. Long story short, I am looking for some information on how the default tmpfs filesystems (/dev/shm, /run, ,etc.) get created. Is the default mechanism for this hard-coded into the kernel or possibly implemented in some operating system service(s)? I'm very interested in how this mechanism works and might be modified.

Thanks!