procfs
The virtual filesystem under /proc where the kernel exposes processes and state as readable files.
None of it is on disk. The contents are generated when you read them. Each process has a directory named for its PID where cmdline, status, fd/ and maps reveal everything about it — that is where ps, top and htop get their numbers.
Beyond processes, the system's knobs live under /proc/sys, adjustable with a simple redirect: echo 1 > /proc/sys/net/ipv4/ip_forward enables packet forwarding immediately. Persistently, the same is done via sysctl. Everything being files means ordinary tools like cat and grep suffice, which is the Unix philosophy applied to system introspection.