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!