All the details are here:

http://www.chromium.org/chromium-os/chromiumos-design-docs/d...

Depending on your device, you unlock things by pressing Power-Refresh; that switches the device to unlocked mode. You can continue using ChromeOS, build your own ChromiumOS, or install another OS.

You can also kill off the bootloader completely. I've never done that before, but I believe the linked doc describes the process. I'm not exactly sure how to install Ubuntu, but the hardware is not going to stop you and never has.

The Pixel-specific instructions are here, though this is for the old revision and I'm not sure what (if anything) has changed:

http://www.chromium.org/chromium-os/developer-information-fo...

Is that an ARM one? You can certainly build Chromium OS (which is Linux), including packages that you want, and install that. It is a somewhat involved the first time, but possible. You probably want something more self-hosting though (like, apt-get dist-upgrade works), and that sounds like it would be a lot of work. There's also Crouton which might get you what you want.

But yeah, ARM booting is consistently crazy and I don't understand how it works at all.