Note that I’m not an expert on this topic. I hope others will correct me if I’m going to say anything blatantly wrong.

Anyway, the way I’ve always understood it is that coreboot is a free BIOS/UEFI replacement (although it does contain non-free blobs). You install it on some memory on the motherboard and so it requires flashing for this reason. It’s not a bootloader. GRUB and U-Boot are examples of bootloaders: this is software that you’d have installed on your hard disks and that the BIOS jumps into when ready.

Libreboot is neither. It’s a distribution, packaging coreboot with other software (I’m not sure if GRUB is packaged as well). It simplifies the task of configuring and installing coreboot on you motherboard by taking care of all the advanced configuration and by compiling a ROM for you (you still need to flash it, though). I think originally libreboot used to strip all the blobs from coreboot, but I don’t know if it’s the case anymore because without the blobs it would only work on a handful of old motherboards.

Correct, libreboot used to remove the blobs, now they try to reduce them: https://libreboot.org/news/policy.html#blob-reduction-policy

There’s also Canoeboot that has no blobs, named in that previous link: https://canoeboot.org/

Damn! I can’t keep up with all the libreboot forks she starts :)

I expected another logo :P

I performed a bit more research and found out that their architecture is really mostly about hardware initing though they can directly load kernel without bootloader

see https://www.gnu.org/software/gnuboot/web/