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Now more than ever, ChromeOS is Linux with Google’s desktop environment
(www.aboutchromebooks.com)
From Wikipedia, the free encyclopedia
Linux is a family of open source Unix-like operating systems based on the Linux kernel, an operating system kernel first released on September 17, 1991 by Linus Torvalds. Linux is typically packaged in a Linux distribution (or distro for short).
Distributions include the Linux kernel and supporting system software and libraries, many of which are provided by the GNU Project. Many Linux distributions use the word "Linux" in their name, but the Free Software Foundation uses the name GNU/Linux to emphasize the importance of GNU software, causing some controversy.
Community icon by Alpár-Etele Méder, licensed under CC BY 3.0
There are 2 ways to do it, either via depthboot(software only, no custom firmware, lots of manual OS prep, 0 risk) or custom firmware(maybe physical, model dependant, no os prep, small risk). For custom firmware you usually have to either bridge an internal jumper, unplug the battery, or build a custom cable, depending on your model.
While it is allowed it's not supported by google.
I would never recommend buying a Chromebook with the intention of replacing the OS unless you're looking for a project or you're getting it for cheap.
TLDR it's harder to replace the OS on a Chromebook than a Mac, and you get none of the benefits of Mac hardware.