Google based Android off Linux. Apple based IOS off BSD. Both of those were the result of collaborative work between thousands of millions of contributors.

Frameworks exist that assist in making apps for either or both, just like they exist to build games for multiple consoles etc.

Meanwhile, Google has now declared that they will require developers register signing key WITH THEM in order to make software that will run on Android (regardless of whether it's installed via their store or another) and has been taking steps away from providing the the necessary codebase under AOSP etc that allow for third-party projects that were based on Android

Google has also transitioned readily to "rent" based services such as YouTube while killing off Google Play Music etc (such actually allowed purchases/downloads of media). Both companies are already heavily investing in generative AI.

Do you think that once they have control of all app signing they'll allow apps that circumvent their advertising or data-harvesting?

This doesn't "make sense", it makes us pay more to a digital landlord who throws around their weight to lock us in further and further while using their increasing wealth to buy up or crush all the competition. We're accepting chains of convenience today in trade for restrictions and exploitation tomorrow.

I can think of one company that holds a dominating market position and has been somewhat benevolent, and that's Valve. They don't buy out competition, they've been active contributors to open-source (to the extent that they've made gaming on Linux actually viable and good), and they often seem listen to their customers in order to improve. They still do take a goodly % of sales revenue from developers who list with them though.