After wondering for a lot of years I recently got a fair idea on what is the difference between stock android, android one and many other custom rom. Now I want to play around with it and was thinking to install Grapheneos. I have a Motorola One Power. Now the question is: say I mess up while installing Grapheneos or say I did install it successfully and decide to switch back to Android One, how do I do it? I tried searching online but couldn't find anything. All I got was to install android one "experience" which is basically just installing a launcher.
One more thing, since Android updates are directly given by Google for all the phones, why do I have to download the OS for a phone from it's manifactrer's website instead of downloading it from the android website? (someone posted a duckduckgo search for what I asked which is kind of what I was looking for, I found a Motorola page for the custom ROM for my device.)
Also, do you guys have any tips before I go on installing? By the way, I am a Linux user if that concerns in any to the question.