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.
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.