When the system first boots, it is running from the initrd, which is a small root filesystem in RAM. In that, there is a script named "init" that performs various functions, including the initial mounting of /proc.
The initramfs-*.img files in your /boot are compressed cpio
archives of that initrd. If you want to examine the init
script, you can extract it into your current directory like this:
gunzip -c /root/initramfs-$(uname -r).img | cpio -im init