"Android is open source".
So let’s take a look at it.
If I buy an Android device from Google today, can I use it just with the open source version of the OS? No, I can’t. Is there any device I can use just with the open source version? No.
So, what is in the proprietary version that isn’t in the open source version?
The Dialer is proprietary (since somewhere between 4.4 and 5.0), the Calendar is proprietary (since 4.4), the Contacts app is proprietary (since 4.4). The on-device search is proprietary (since 3.0), the homescreen app is proprietary (since 4.4).
The drivers are proprietary, the firmware is proprietary, the libraries to use recent openssl on older versions are proprietary, the only way to do wifi geolocation is proprietary, the only way to run in background is proprietary.
Android is as much open source as macOS is: Darwin is open source, and webkit is open source, but everything inbetween is proprietary.
I suggest you read https://arstechnica.com/gadgets/2013/10/googles-iron-grip-on... to learn about why your statements about Android are deeply misleading