The stat() family functions for sure, BSD Sockets, user account, covering everything from "process environement" to "filesystem" functions.

"FAQ about lsof"[1] is very instructive.

[1] ftp://lsof.itap.purdue.edu/pub/tools/unix/lsof/FAQ

On Linux, it probably uses files under /proc. For other systems, I don't know.

strace it and see. As the other poster notes probably open() call to procfs or stat()

If this sort of thing interests you, I cannot recommend https://www.nostarch.com/tlpi enough. The hardcover is worth every penny.