Android isn't a "totally open platform". By default you can only install apps from the Play Store and Google exercices a fair amount of editorial control over apps.
On the latest android its a huge pain to install fdroid. The only way I could find was installing the system fdroid package through the recovery thing because android doesn't just allow you to install 3rd party apps, you have to get an existing app to request permission to install 3rd party apps and none of the default ones do so unless you already have fdroid you can't install it. Really unethical.
On my Pixel 3: Settings / Apps & Notifications / Advanced / Special app access. From there you can whitelist, e.g., Chrome to allow the installation of 3rd party apps.
IIRC Chrome even asks you if you want to enable that setting once you download an APK file.
I didn't have chrome on my phone and the default android browser doesn't request permissions. I used to install it from the downloads app thing but that doesn't seem to work anymore because of these new changes.
You can run stock Android without any Play Store and no/minimal/neutered Google services by installing custom ROMs. Apple doesn't allow this with its hardware. More importantly, you can bypass any centralized app store and install your own APKs, such as from repositories like F-Droid. IOS doesn't allow you any such freedom.
