Android is the world's leading mobile operating system. It also has one of the most active developer communities. Since Google has made Android open-source, it means the source code of the operating system (OS) itself is available to developers, and anyone can make their own version of Android if they know what they are doing.
This is the reason why the Android OS varies from Google's Pixel devices to Samsung's OneUI and Xiaomi's MIUI and everything else. This also means that individual developers can create their own custom versions of Android, also known as custom ROMs. Developers can create these custom ROMs and add new features and functionality that aren't available in stock/vanilla Android
A custom ROM is essentially a firmware based on the Android source code provided by Google. A lot of people prefer custom ROMs because of the functionality they offer, and the ability to customise many things on the phone.
Another big reason why many Android enthusiasts flock to custom ROMs is that their devices stop getting new software updates after a couple of years and having a custom ROM offers them the ability to improve functionality and get their hands on the new software.
