The AOSP (Android Open Source Project) is the software stack for the Android OS. It is led by Google. The fact that Android is an open source software has led to many custom Android variants. It also is a big assurance of Android's safety in terms of privacy.
Android gives users the ability to define which apps perform which action. For example, if an Android device has several browsers installed, the user can choose a default one which will start every time a link is for example opened from another app.
Even though Android is open-source, it is near unusable without Google Apps and a Google account. For example, it's impossible to download apps from the Google play store without a Google account unless the user is sideloading the app from 3rd party source such as APK Mirror.
Most Android phones seem to be released on a "Let's release a few software updates until we release the next phone, and never push out an update again." For example, In May 2014, 8.5% of phones were Kitkat, 60.8% were spread out between three versions of jellybean (with bias towards o...
Android products might appeal to the more technically capable, people prepared to invest in considering their product, who like a particular device form factor and / or who already uses a lot of Google’s online product suite.
It's engineered to be designed. Design means choice. A collection of best options, components and programs chosen by highly experienced curators/designers/engineers are installed by default. No need to customize. It's best by default.