Neither has BIOS in the absolutely conventional sense that we know and love.
Best to ask the question separately for “laptop” and “phone”.
I think for the “phone” there is no BIOS. Period. The bootloader is uboot. It’s normally on the eMMC drive but can be downloaded from a host computer connected via USB (uuu). uboot would then normally continue the boot by loading the kernel from the eMMC drive. Citation: Librem5 sd and boot order
However none of that tells you how Purism achieves that at industrial scale.
Adding: PS That means that in some sense “flash” is a misnomer for the phone. There is no flash to “flash” in the sense that it applies to your average PC, no separate flash.