I think in this case it can be approximated as 'largest market'

I'd wager there are more people paying for software for their smart phone than any other platform they use.

Having my credit card already is an overwhelming advantage for the Apple App store and for Steam. I won’t say it is impossible to overcome, but I think I could count on my fingers the number of instances where I, like, typed my card into a website to buy anything, in the last decade.

Yes, but they are mostly paying little or nothing. How much did you spend on phone apps this year? And ads pay a pittance, unless you have massive scale.