Hang around any part of the internet that talks about smartphones long enough, and you'll see people arguing the merits of Stock Android. On one side you have folks with a Pixel phone, Google's Android the way Google wants it, and on the other, you have someone with a Galaxy phone; Android the way Samsung wants it. These two phones offer a very different user experience in some ways but are also very similar. When it comes to the "Android" part, more similar than most people think; and neither is Stock Android because Stock Android isn't what most people think it is.
It takes a lot of software to make a phone work. There are all the parts we can see and tap on and swipe away, but behind everything you see on a screen is software that runs the apps and keeps the Wi-Fi working and keeps track of your location and does all the other things we take for granted when we\re using it. The operating system on your phone is incredibly complex and includes parts from the company that made it and parts from the companies that made the individual hardware components like the display or the processor. Android is one of those parts, not the sum of those parts. It's complicated.
Android is one of the parts of your phone's software, not the sum of those parts.
We use the word Android for several things, including one thing that's it's really not -- an operating system. On a phone like the Galaxy S9, Android is really only part of an operating system along with parts from Samsung or parts from Qualcomm and other parts also from Google. On a phone like a Pixel 2 XL, Android is also only part of an operating system along with parts from Samsung or parts from Qualcomm and other parts also from Google. All these companies need to work together to make your phone turn on and do things, and the one company that makes it and sells it has to manage the parts and assemble them into software that keeps it all running. A Pixel 2 doesn't run Stock Android. Neither does a Galaxy S9. Because Stock Android is the software that serves as an application framework and a hardware interface that lets that framework communicate with all the parts it needs to keep itself running. Both phones run Android, but the operating system is better described as Android-powered. And for a while now, that Android portion of the operating system on almost every phone has been nearly identical. They have to be because Google says they have to be in order for free access to Google Play and all the Google services.
It wasn't always that way, but early on someone rightfully decided that if you wanted all the phones to be able to use all the apps there needed to be some pieces that were uniform no matter who made that phone. This means the "Android" part of any phone is the same as the "Android" part of any other phone (or Chromebook or watch or television) that has the same version. It's also a thing that we can't see because of the other software on that phone or Chromebook or television. Those parts can look very different. The Galaxy S9 may have the same framework (with extras added that aren't part of "Android") as the Pixel 2, but visually it doesn't feel that way. This is the part most of us are talking about when we say Stock Android.
Having different phones that can all use Google play is what's important for "Android", not the label we put on them.
Thing is, neither the Pixel 2 or the Galaxy S9 are running it. If you go back through the history of Android, somewhere around the Galaxy S and Motorola Xoom Android from Google diverged from the product of the open-source code and into the thing that Google wants it to be. You can still build an Android-powered operating system from the open source project, but it certainly won't look like anything you can buy today, even from Google. The actual Android framework would be the same no matter what or who built that software but the interface doesn't have to and won't. None of this really matters anyway. It's trivial to make a Galaxy S9 and a Pixel 2 look the same when it comes to the user interface The extras that Samsung adds are easy to hide and the things that Google doesn't include can be added in from the Play Store. Differences like a better display or compatibility with Samsung Pay have nothing to do with Android software. These two phones look and feel very different for plenty of reasons, none of which are Android.