Your logic is incredibly flawed.
First of all, some of our players aren't good enough to play for City, because they are too young? Rico Lewis played a big part in City's success last season, at 18 years old. If Pep had Mainoo, he would be getting significant game time there as well, no doubts about that.
If you watch him play, and actually understand what you're watching, you'll know how good he is, despite playing in a shit team. And I don't see how you can just dismiss the performances he's shown at United with "well, he's 18, not 22, so he's too young to be as good as a City player"
The only difference for City is that they don't need to rely on 18-20 year olds to be difference makers, like we do, because of the quality of their squad. They can wait for their Lewises, Bobbs, Fodens, etc. to be 20-21 before they play 50-60 games a season. But the ability for obvious future stars like Mainoo is there already. He'd probably get 20-30 games per season for them right now, but not down to his ability, but due to the things I've just listed: basically they can afford to manage them better.
You also don't also realize that most of City's players aren't demigods, they are just quality players put in the best possible tactical setup and environment, where their weaknesses are masked and they are surrounded by other great players.
That's why I listed two examples: switching Mainoo with one of their midfielders, and/or the two goalkeepers yesterday wouldn't have changed anything. Onana or Mainoo would've performed well for City yesterday, and would do so week in week out, whilst Ederson or Rodri/Bernardo etc. wouldn't be able to carry this United team on their own, maybe other than De Bruyne. But even De Bruyne would look worse if you just put him in our team and made no other changes.
Such an obvious thing that's easy to understand, except on RedCafe where the general consensus is always that we have 3 or 4 good players at best, which should have us finishing around 15th every season, but sure, keep ignoring reality that this team as a whole, is actually way less that the sum of its parts.