Apple and Google are responsible for 99% of all mobile software distribution[1]. They both monopolize the mobile app distribution market, as well as the mobile app payment markets.
Both firms leveraged their dominance in the mobile OS market to dominate the mobile app distribution market, and then they leveraged their dominance in the mobile app distribution market to dominate the mobile app payments market.
When talking about monopolies, the working definition is firms that have significant and durable market power such that they can set prices and exclude competitors[2]. Both Apple and Google fit that bill for the mobile OS, app distribution and payments markets. Whether you use the words monopoly, duopoly or cartel to describe them doesn't really matter, because all of those terms are accurate descriptors.
Apple makes very popular computers in their iPhones, but other phones, tablets, laptops, gaming consoles, etc all do the same stuff.
Eg. When your phone is broken, you can still call people over your laptop, and you can access an internet browser from basically anything