Can you buy an alternative product and have nothing to do with Apple's store? Yes. They aren't a monopoly.
Are they a big player? Yes. But they aren't a monopoly. If you don't like it go somewhere else. Vote with your dollars and impressions.
They have more than 50% of global app sales revenue; Apple's App Store revenue is about 80% higher than Google Play. They also have about 57% of the "premium" ($400+) smartphone market. That has a strong impact on the ability for developers to create and sell apps.
Not to say that Google is innocent either; they've started adopting some of Apple's practices as well. Unless you're a masochist, going outside the Apple/Google ecosystem is not really a viable option.
https://techcrunch.com/2019/07/03/app-revenue-tops-39-billio...
https://www.gsmarena.com/counterpoint_flagship_smartphone_sh...