It depends on the model. If you have an eMMC model of Jetson, then there are a lot of small partitions other than the rootfs. Within those are a binary kernel and binary device tree (they also are signed; the default sign is NULL). There is something similar going into QSPI of an SD card model (only dev kits will have SD card model of module). In theory there would be a binary kernel and device tree in the QSPI which would be called, but I don’t know of anyone who has actually tested that. Maybe an FDT entry is required and a kernel listed in extlinux.conf? Unless someone knows for certain I hesitate to say it, but probably there is a device tree loaded from QSPI (which would be added during flash). This file would not be manually picked, it is just a case of priorities. For eMMC the FDT and LINUX lines of the extlinux.conf have highest priority if security fuses have not been burned; failing those loading the load switches over to the signed partitions. “Probably” it does this on SD card models with QSPI, but maybe not.
Usually one would get a full serial console boot log to find out that answer (make sure you remove any “quiet” key word from “extlinux.conf”).