We call this filesystem initrd –> Initial RAM Disk (older style, used to be formatted with EXT2) or RAM Filesystem (this is different, as this filesystem can grow and shrink), used to provide drivers and support for mounting the system’s real root filesystem.
Inside the root filesystem, there is a program that the kernel runs first.
This is init from initrd and the kernel can look for other names as well.
The First Process (from disk)
When init from initrd terminates, the Linux kernel starts init again. This time from the real filesystem, which is commonly on the disk.
If you have an init that is a soft link to something else, the kernel will run that instead.
init is usually a softlink to systemd now.
This is responsible for starting up services such as daemons, like a web server.
