Android/iOS is a duopoly as pervasive as Visa/MasterCard. The ither examples only to a much lesser degree.
For example my phones use Android, but I do not have a Google account.
There exists no legal action that could be done by Google and that could influence anything that I am using my phone for.
On the other hand, if Visa and Mastercard would decide to not let me buy something, that would be very bad for me, because I buy almost nothing with cash and most things that interest me could not be bought with cash anyway, because they cannot be found in the shops from my close neighborhood.
So the monopoly of Visa and Mastercard really matters and it is very hard to circumvent. That of Apple and Google, much less so.
Society is not math or computer science where existence of one counterexample makes things invalid. Society is a thing of ratios and percentages. If something is almost impossible for most people, we have to treat it as impossible even though it is possible for some - otherwise bad actors get away with terrible stuff on technicalities.
(I for example pay everything with cash or wires which are free and instantaneous in my country - I use card only once in a while to withdraw cash - but it does not invalidate Visa/MC duopoly any more than your special Android that only one in ten thousand people can use either)
I do not need extra apps from the Google Store, because all the phones that I buy do the things that I need, e.g. voice calls, SMS messages, Internet browsing, photo/video recording, document reading, etc., without needing to install anything else. Things like Google Maps can be accessed in the browser, there is no need for an app.
So it is not difficult to avoid Google.
To avoid Visa and MC, one would need exceptional connections, because most online shops do not accept other methods of payment and it would not be easy to convince them to create a special method for you.
I choose wire transfers whenever I can instead of credit cards. For payments within the country there are no fees, but for payments to other countries, the fees are larger than for credit cards and the vendors are less likely to have wire transfers as an alternative method of payment.
This is why personal anecdotes are meaningless for these topics.
If you agree that the required ability is "use my portable cell-connected computing device to exchange messages with others on their similar devices", then absolutely, I agree that there's a duopoly there (I'm ignoring the possibility to use alternative app stores on Android, because it doesn't seem particularly central).
But if you instead think the required ability is "be able to exchange messages with others in more or less realtime", then the duopoly in the portable cell-connected computing device world is of much less importance.
For years, because I don't have a cell phone, I was excluded from communications taking place via SMS (there were a few 1-way linux-accessible gateways, but they didn't really change anything there). But I didn't feel that my ability to communicate with others was really particularly impaired, and eventually Telegram (and others came along) that I could use from a desktop, and I was more or less back at parity for "real time communication with others".
So whether or not the android/ios duopoly matters in this sort of instance depends on precisely what functionality it is that you think people have a "right" to ...
That last one, too, is probably a big part of the reason for the classification. There are more Visa/MC debit cards out there than their are credit cards.
2. If you look at transaction, Amex and Discover may have higher processed transaction value of something like 15% in US. But if you look at transition volume, Visa / Master are 90%+.
3. Both are Credit Card only. Debit Card is where the majority are using for Payment.
3. Consider there are no other electronic / digital payment method that is as ubiquitous as Visa / Master payment network ( ignoring Asia ) and they are the baseline method for e-payment, as well as an extreme high level barrier of Entry. The two are considered as Duopoly.