What is preloader?

Hi. guys.

I don’t quite understand the relationship between bootrom and the preloader.

As far as I know, the preloader is stored on the eMMC (/dev/mtd0). However, even if I delete all of /dev/mtd0 with the command below, the preloader still works.

$> mtd erase /dev/mtd0

The attached image shows the result of a reboot after I deleted all of mtd0.

mtd0 = nand

mmcblk0 = sd

mmcblk1 = emmc