They are all considered “natural monopolies”. They are also regulated on the municipal level. Do you now want each city to regulate apps? Why stop there? Why not regulate what console makers can sell? every physical store can sell?

In my country >90% of smartphone users have Android. Yes I would call that a monopoly. Yes I think monopolies should be regulated. Not at the city level since that's ridiculous and sounds like a strawman. That comment was just an example of monopolies that need to be regulated.

Isn’t the whole beauty of Android that it is “open” and that you can sideload apps and you are not beholden to a “walled garden”?

As Epic found out, not if you want to remain on Google's good side. They were facing sanctions from Google due to their sideloading of Fortnight onto Android phones and tablets.

What “sanctions” were they facing? They were called out because their app was a security nightmare.

https://www.cnet.com/news/fortnites-battle-royale-with-andro...

My understanding is that the "vulnerability" here also exists on every computer or other device on which you download an installer to a public folder--any application can watch that folder and silently replace the file with another when the download completes. There's nothing special about that. Android has a secure internal storage area--but internal storage is often limited. It's gotten better, but my previous phone only had 8gb for both the OS and data to share.