AOSP, which is hardly ever run on its own, is open source.
Things like the Play store are entirely closed source. A majority of the API shipped by the Play store are closed source. Many hardware drivers are closed source. The majority of the operating system is Apache licensed, allowing a handset maker to ship modified components without publishing their changes.
Many vendors also prevent the user from installing their own custom ROMs, so even a sophisticated user may not be able to run AOSP on an arbitrary device.
Google Play store is a very small part of Android. Just the fact that vendors can ship modified components is a testament to Android being open. Even Huawei shipped a modified version of Android with Google services such as the Play store removed due to sanctions.
Also in comparison to iOS, Android is a heck of a lot more open. Apple can ship whatever they want and no one would be none the wiser. Plus you can't install whatever software you want on an Apple device, and are restricted to what Apple thinks is acceptable.
Things like the Play store are entirely closed source. A majority of the API shipped by the Play store are closed source. Many hardware drivers are closed source. The majority of the operating system is Apache licensed, allowing a handset maker to ship modified components without publishing their changes.
Many vendors also prevent the user from installing their own custom ROMs, so even a sophisticated user may not be able to run AOSP on an arbitrary device.