Linux has a virtual filesystem named proc; the default mount point for it is /proc. The first thing to realize regarding the proc filesystem is that its content is not on a non-volatile disk. Its content is in RAM, and is thus volatile. The files and directories you can see under /proc are pseudo files that have been set up by the kernel code for proc; the kernel hints at this fact by (almost) always showing the file's size as zero:
$ mount | grep -w proc
proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
$ ls -l /proc/
total 0
dr-xr-xr-x 8 root root 0 Jan 27 11:13 1/
dr-xr-xr-x 8 root root 0 Jan 29 08:22 10/
dr-xr-xr-x 8 root root 0 Jan 29 08:22 11/
dr-xr-xr-x 8 root root 0 Jan 29 08:22 11550/
[...]
-r--r--r-- 1 root root 0 Jan 29 08:22 consoles
-r--r--r-- 1 root root 0 Jan 29 08:19 cpuinfo
-r--r--r-- 1 root root 0 Jan 29 08:22 crypto
-r--r--r-- 1 root root ...
