The bootloader (u-boot) starts either from eMMC or USB (uuu). Bootloader cannot be launched from an SD card.
u-boot then loads the kernel, dtb and initramfs, also either from eMMC or USB (with uuu). Loading those from an SD card is theoretically possible, but will require drivers in u-boot since the SD card reader is a mass storage USB device behind a USB hub.
Once the kernel is loaded, it can mount the rootfs from any device. I’ve already successfully booted operating systems on Librem 5 with rootfs on microSD or external USB drive this way.