It seems that eMMC has built-in boot partitions. eMMC devices are not just "raw storage" (like SD-Cards) they have special reserved hardware partitions defined by the JEDEC eMMC standard.
These include:
boot0 – a small partition (usually a few MB) reserved for bootloader code.
boot1 – a second small boot partition, often used as a backup.
RPMB (Replay Protected Memory Block) – a secure partition for keys and counters.
The SoC’s boot ROM (first-stage loader) can be configured to read directly from boot0 or boot1 when powering on. Also, these partitions are read-only to decrease the risk of having corrupted boot code.
Beware that they’re not used by the Linux filesystem unless explicitly written to (usually via dd when flashing U-Boot).
