the SoC will likely look in EMMC, then SD, then SPI for devices, then execute at a specific address when it finds something like how BIOS looks for the 512 bytes at the start of a GPT volume.
My pinebook has towboot on the SPI rom. It boots towboot and towboot knows how to load a kernel over NVME. Then its basically at the kernel already.
the SoC will likely look in EMMC, then SD, then SPI for devices, then execute at a specific address when it finds something like how BIOS looks for the 512 bytes at the start of a GPT volume.
My pinebook has towboot on the SPI rom. It boots towboot and towboot knows how to load a kernel over NVME. Then its basically at the kernel already.