Side-question, but ... is this true?
I have a bunch of Amazon tablets, though they say "FireOS" instead of Android, that don't come with the Google Play store.
And in China, there is no Google Play store available at all.
Partially true. Android is open source and anyone can leverage it to put on their devices which many of the Chinese manufacturers do and Amazon has done. However what isn't included in that is
Google Mobile Services, aka "the best of Google". GMS is what provides the Play Store, Google Chrome, YouTube, Mail, Maps, Photos, Messages and
a number of support APIs including location services, Google Pay, Wear OS, enhanced security/auth features and some ML features.
Amazon have their own services that provide many of these but the coverage isn't great and there are quirks when dealing with them compared to the Google versions (keeping in mind something as simple as location services differ between the distributions). Developing for FireOS is extra testing for a very small base and is why their app store is light compared to the Google Play Store.
Many of the Chinese phone manufacturers bundle GMS for their international devices however in China almost all of those services are blocked anyway and they have their own versions. In many cases apps like WeChat also provide the app experience on their own providing those services.
Part of Epic's suit against Google includes a unique claim that
Google threatened OnePlus over plans to ship Fortnite with their devices. Similarly the same article suggests that LG had a contract “to block side downloading off Google Play Store this year.” If proven true, this shows Google engaging in the same behaviour that got Microsoft into trouble with Internet Explorer (part of the Netscape claims were that Microsoft interfered with OEMs looking to pre-install Netscape).
Ok, but how is Google interfering in the smartphone market by offering Android to smartphone OEMs? Smartphone makers are free to go with Android or a completely different OS.
If that's your definition of "interfering" then Apple must be interfering in the app market by offering (actually, requiring at a cost in order to reach iOS users) developers to use the App store to market their own apps.
Google interfered in the smartphone
operating system market resulting in the alternative operating systems all disappearing. Back when the iPhone launched in 2007 there was Symbian from Nokia, Microsoft shipped Windows Mobile and later Windows Phone, Blackberry had their own OS and Palm also had their own operating systems, including eventually WebOS. Android doesn't hit the scene until 2008 and within three years becomes the dominant operating system with over 50% of the marketshare eviscerating Symbian (the market leader), Blackberry and Windows Mobile/Phone (Phone 7 came out late 2010).
Ultimately Google gave away the basic operating system and all of the former Windows Mobile based OEMs shifted to shipping Android devices (and why wouldn't they, at the time Google charged no fees compared to
buying Windows Mobile for up to $15 per device). Part of I believe some of the EU concern is that Google over time has moved functionality out of AOSP (or let it remain stagnant) as Google's Mobile Services bundle is expanded with extra functionality and features. The EU being the friendly chaps they are
have fined Google over illegal tying relating to GMS as well and
more recently reports of investigation over voice assistants.
Smartphone manufacturers are at this point stuck with only one option: Android. Mozilla's Firefox OS failed, WebOS was a late entrant and failed now relegated to running TV's, Microsoft's Phone OS is dead and Nokia's Symbian and Maemo platforms are dead. Even on the open source side the OpenMoko work fell away to Android. The challenge for the OEMs is that much of what consumers associate with Android is provided by Google Mobile Services which means they're tied to licensing that or shipping devices where something as simple as Maps doesn't exist out of the box. A customer is going to return that quick smart.
Android undercut the smartphone operating system market and gave Google a monopoly in that market. The challenge in attacking Apple as being a part of the "duopoly" ignores Google's role in removing all of the other competition in the space. Attacking Apple for being a "monopoly" is awkward because they are the only remaining competitor to Google.
This video on YouTube on smartphone OS marketshare I think demonstrates just how quickly Android killed everything else on the market and how Apple's iOS has slowly gained the marketshare it has over time. That's no mean feat when you consider their devices are generally more expensive than most smartphones on the market so they're not winning by undercutting the market, they're winning by being the better product.