I have limited knowledge and can't generalize, but here's what I do know about one specific device.
That's the Nook Touch with a TI OMAP3621 processor.
The OMAP has ROM directly on chip for a very simple bootloader.
Depending on how the boot_sys pins are configured it can load from NAND, SD card, UART or USB.
It will load a primary bootloader (mlo, x-loader) which loads the secondary bootloader (u-boot.bin) which loads the kernel and ramdisk image(s).
Some devices do have a locked ROM bootloader which will only load signed primary bootloaders.