In sysvinit I would have an initscript, populate-volatiles.sh, that reads a volatiles file and creates the volatiles. What performs that function in systemd? Is there a systemd unit that does this? What is it and how do I configure it?
The unit I was looking for is systemd-tmpfiles. It creates volatiles/tmpfiles. Helpful docs here: https://www.systutorials.com/docs/linux/man/8-systemd-tmpfiles/