Real talk: sideloading is allowed on android in the most maliciously compliant way possible.
Google restricts what other app stores can be included with devices that ship with play services
User-sideloaded app stores can't auto-update apps
Play protect will flag any app that the play store has hashes of, but was installed by another app store. (Developers cannot, for example, upload a list of valid hashes for their apps to Google to prevent false positives here, effectively making other install routes appear as malware if they're different.)
This is from wikipedia: "In law, a monopoly is a business entity that has significant market power, that is, the power to charge overly high prices, which is associated with a decrease in social surplus." As a side note, I find it really distasteful when people say, "It is not a monopoly," because it adds nothing to the conversation, and is almost always wrong.
And tossing a Wikipedia quote at me doesn't make what I said incorrect, either. Google allows (and has) numerous third-party app stores on their devices, some of which are completely free. The app store that Google themselves operate also has lower fees and looser restrictions than Apple's own store. Android is also an open-source platform that allows sideloading of thirdparty applications. Most device manufacturers heavily modify it to suit their needs.
Anecdotally: I have used Android phones for most of my smartphone-owning life and have recently been using iPhones for the past few years. Apple's app situation is an absolute joke. It is so incredibly expensive and locked down that the app store is almost completely useless unless you are ok with paying monthly sub fees for even the most basic of apps. Apple is literally fighting a court battle with the EU right now over their app store in which the court branded it as a monopolized service.
Google allows (and has) numerous third-party app stores on their devices, some of which are completely free. The app store that Google themselves operate also has lower fees and looser restrictions than Apple’s own store. Android is also an open-source platform that allows sideloading of thirdparty applications. Most device manufacturers heavily modify it to suit their needs.
There is nothing about any of that which has anything to do with whether Google is using their market power to unilaterally charge more than they would in a fair market.
It is not a monopoly. They have Apple as a major competitor
Thank god! Where can I download the Apple App store on to my Android phone? I can't? Then it's irrelevant to this conversation around Google's monopoly on Android.
Google allows sideloading within their own ecosystem.
As @logicbomb points out, just because a ecosystem is open, doesn't mean a monopoly doesn't exist. All the other stores are pretty niche and Google controls 90%+ of the market, so by definition it is a monopoly. A monopoly on it's own isn't illegal or even bad, and we have to dig in further to determine that. As you pointed out, it's pretty clear-cut that Apple has a forced monopoly where users have to actively work against the system to load apps outside of Apple's ecosystem. While Google's case isn't as clear many have argued that Google's Android has kneecapped alternative stores like Amazon's, possibly in anti-competitive ways.
I personally love f-droid, but Google does not make it an easy process to sideload!