No rootfs is bootable in and of itself. It is just the file system. An SD card is not bootable without other content.
Boot content exists in the QSPI (for eMMC models it is the eMMC) memory. Earlier releases had part of the content on the SD card, more recent releases moved all of that to the QSPI.
It is possible though that the kernel or device tree can be retrieved either from the image’s “
/boot”, or from a partition in QSPI. A given software release within the QSPI is generally the same for a range of releases, but more recent releases cannot use the QSPI of the older releases (I don’t remember which L4T version this split occurs at, perhaps someone here knows that answer).
In the case of content for the kernel or device tree coming from the rootfs it is the flash software command line which determines which kernel, device tree, and
extlinux.conf is copied into the image’s “
/boot” prior to generating the rootfs image.
The QSPI of the SD card model dev kits has its boot content flashed with JetPack/SDKM.