Hard to know since Amazon doesn’t release sales numbers.
Either way Apple isn’t doing anything to shut down tablet competitors other than making a great product at a price people seem to be willing to pay. There aren’t tons of lawsuits or requirements that retailers can’t sell competing devices.
For example, Apple only has 10-40%[1] of the smartphone market (i.e. market share), but they capture north of 80%[2] of the physical device sale profits. There have even been years where they captured more than 100% of the profits[3]..
The same goes for the app store: iOS owners are often more likely to pay for apps, but Android developers make up this difference by including ads and tracking, which isn't 'consumer spend' but still makes money.
[1] depends on who you ask
[2] https://appleinsider.com/articles/18/04/17/apple-grabs-86-of...
[3] https://www.macrumors.com/2016/11/04/apple-smartphone-profit...
Apple has 100% market share in the iOS market. Due to that fact they’ve got exclusive control over a significant percentage of the population - consider that customers cannot install another OS on the hardware they’ve bought and they cannot move their iOS apps on another Android phone either.
To make an analogy consider that you’d complain about your local electricity provider having a monopoly and I’d tell you that you can always move to another city or country, there, problem solved, it was your fault for staying there.
And Linux has 100% market share in the desktop Linux market. But it probably doesn't count as a monopoly.
I do believe the significant market share of Apple smartphones is an issue, but as far as I know they haven't managed to make it a monopoly yet.
And if we're looking at e.g. "tablets", I wouldn't be surprised if they had something like 90% of the market, in some countries.
It's funny how Apple is simultaneously depicted "non-dominant player" and "the only mobile platform where you can make money", sometimes by the same people.