I don't see Amazon as a monopoly, but as far as Apple goes, they currently hold the majority of phone sales in the US. A big reason for that is that they have a history of artificially preventing you from using certain device features with non-Apple products or services. iMessage is a good example as it took an order from China for Apple to add the RCS messaging standard that Android phones have had for years. Another example is that, while music apps on Android can use Google Assistant features, only Apple Music can utilize Siri features on iOS.