Android's main feature is that it is open source. Including the kernel, UI, libraries, and key apps, the full operating system's source code is publicly available.

Therefore, anyone can create Android from the source code and flash it onto a compatible device (with the right technical skills). Different groups - some commercial and some hobbyists - have been able to create alternative distributions of Android due to this flexibility. In common parlance, these are known as custom ROMs, but a better term would be custom firmware.

We should pause and assess the enormity of the job ahead before diving into the murky world of building custom versions of Android. If you are not familiar with coding, Linux, or Makefiles, then this is not the course for you.

The Android operating system is a complete package. It contains a number of different components. Android's complexity and usefulness weren't created overnight. Therefore, any customizations you wish to make need to start small. The creation of a radically different alternative Android distribution will require a lot of time and effort.

Making your own custom Android ROM can be rewarding if you are familiar with writing code and know a bit about Makefiles and compilers. A custom Android firmware could theoretically be built for any computing device that was capable of running a modern operating system. We will, however, limit our efforts to developing Android for Google's Pixel devices to make life easier.

You must be familiar with Linux in order to build Android. The terminal will be used a lot, and you will have to be comfortable using shell commands. Linux virtual machines might work, but I recommend that you use a dedicated PC.

As a minimum, you'll need 400GB of disk space, and no less than 16GB of RAM would be ideal. An advanced 64-bit processor will be necessary.

According to Google, building Android on a 6-core machine with 64GB of RAM takes about five times as long as on a 72-core machine with similar RAM. When I tested it, I foolishly used hardware from a decade ago on a dual-core machine. Approximately 24 hours passed!

Be patient. Android development takes time. It can take hours to sync the source repository (depending on how fast your Internet connection is). And a complete rebuild takes many hours to complete.

The build might take up to 20 minutes even after you make a minor change. This is dependent on your hardware. You won't be able to use your new Android version right away.

Google services are not included in the Android Open Source Project version. So you won't be able to get access to Google Play, YouTube, Gmail, or Chrome. These 'gapps' can be flashed onto your custom firmware, but I will leave you to find out how to do that. 

To build a custom ROM, follow these steps. To build your own version of Android, download and build the Android Open Source Project, then modify the source code. 

Documentation about building AOSP is provided by Google. You should read it again, then reread it. Be careful not to skip steps and don't assume you know what's coming next.

Set up a build environment - which includes installing the right development tools. The 'Repo' tool is used to grab the source. Some drivers can only be obtained as proprietary binaries. the 'lunch' tool allows you to select a target. Use 'm' to start the build. Flash the build onto your device — using ADB and fastboot.