The Android operating system many of us use every day is often referred to as open-source software. That means its code is publicly available and can be endlessly modified by anyone. While those statements are indeed true, the vast majority of Android devices today aren’t, in fact, running pure and open-source Android. You can thank Google Mobile Services (GMS) for that.
In a nutshell, the open-source component of Android is the Android Open Source Project (AOSP). GMS, on the other hand, lives on top of AOSP and provides much of the nice-to-have functionality you may have come to expect from modern-day Android.