For x86 chromebooks there's https://mrchromebox.tech/ to replace the firmware with a normal UEFI-booting coreboot, and then you just install whatever you want on it because now it's basically a normal PC.

For ARM chromebooks I'm only aware of https://archlinuxarm.org/ and that's still pretty patchy; I hope that something comes together (or that it already exists and I just haven't found it yet).