How is system-as-root mounted in Android 9?

In Android 9, system.img consist of system and ramdisk. So kernel will mount system.img directly and implement init program. Next, init will parse init.rc file and mount remaining partitions based on fstab file. But, there is some thing in fstab file that I don't understand :

/dev/block/by-name/system      /      ext4    ro,barrier=1

Here, system is mounted as root in fstab file. I thought that system.img is mount before to execute init program but here, system.img is mouted again. Can someone explain this for me?