Per user Amazon is most profitable, closely followed by iOS then Google but install base is the opposite.
As far as I understand the main source of revenue for free games is from advertisements, nothing else 
You are generalising a bit too much now.
Think of games like Candy Crush Saga, Clash of Clans, Clash Royale, etc. Those are among the top grossing games on the mobile platforms and they don’t have any ads. For instance, an article by PockerGamer pointed out that Clash Royale has generated over 2 billion dollars in two years.
Now, these games are at the top of the food chain. They are extremely well designed and refined, plus they each have a marketing budget in the tens or hundreds of millions of dollars. So your game will most likely not be comparable to them in that sense.
The most important thing to really understand is that people like what they have and that includes their money. Making people give up their hard earned money is difficult. Think about yourself and your spending behaviours. How much do you spend on an average free to play title? For mobile games, I don’t think I’ve ever spent more than 5$ or 10$ and even that was after some serious pondering. For some free to play PC games that I’ve played for years, like Heroes of the Storm, I’ve easily forked at least ten times that.
In order to generate reasonable revenues from IAPs, you have to think of your games as a service. Something that will keep players captivated for months or years to come. Does your game have that hook that makes its players want to pay for them IAPs? Do they actually benefit from making IAPs in a meaningful way? If the game is just another indie title, then the answer is probably no and in that case your main source of income is probably ad revenue. Still, even with ads, in order for them to have any effect on your earnings, you need to have a larger active player base.
The breakdown of players who use IAPs is very bleak. At best, my team’s projects have had 3.4% of the users pay at least 1$ and at worst it has been 0.6% on iOS (Android figures are generally worse). But then again, the more a person has paid, the more likely they have been to play the game for longer and the more frequently they’ve made purchases. In other words, these users found something they were willing to pay for and they did. Once a person has made a purchase once, they are much more likely to make another one, etc.
For hyper casual publishers like Ketchapp, Voodoo, etc. you are correct, the majority comes from bombarding users with ads. IAPs for games that have play times measured in seconds is not really a thing.
However, as pointed out above, the big income comes from very sticky games that keep the player hooked. There MUST be some form of competition in there that motivates players to beat other players and pay to gain a competitive advantage.
Just look at the top grossing charts as this is derived from IAP revenue not ads. On the other hand the top free charts will all be dominated but ad income in one form or another.
When I was doing free-to-play games with IAPs for additional content, the conversion rate was about 1-2% (i.e. out of every 100 free downloads, 1 or 2 would upgrade), which equated to about a 50/50 split in income between IAP and ads.
So you need your install base to be in the high six-figures or seven figures for that 1-2% to result in a decent return.
Fortunately I was able to achieve over 3m downloads, but that was a good few years ago when the market wasn’t so saturated and apple gave indies a chance with the new releases tab.
95% of income was from iOS, very difficult to make much impact on Android.