Monopolies aren’t illegal per se in the US if they were acquired lawfully. What’s illegal is using monopoly power in a prohibited way. However, collusion (even tacit collusion) can be illegal even if there’s no monopoly by a single company, e.g., market sharing[1].

Note: Not a legal expert, this is just my understanding.

[1] https://en.m.wikipedia.org/wiki/Dividing_territories

They both carefully added up all their costs and each independently arrived at a fair 30% fee, even though entire physical supply chains can undercut this percent.

This happens often without direct collusion. You see it commonly in retail. Not sure anyone can outlaw unspoken pricing standoffs like this.

They can do it indirectly by breaking them up.

There are already alternative app stores on Android (perhaps most notably, Amazon) and that hasn't impacted their 30%... so what exactly are you expecting?

These alternative App Stores are heavily restricted when it comes to updating or even just installing apps, unless they were installed into the system and the system was patched by the OEM.

They are different markets. If you have an iPhone you must use Apple's store, Google's store is not competition for that market. For Google, use of the store is not required, but there's a risk to side-loading because ideally there would be competing stores that also do quality control and security audits, and there aren't. So it isn't a duopoly, it's one absolute monopoly and one near-monopoly.

The legal definition of a "market" is always going to be somewhat subjective and arbitrary. I doubt whether a US federal court would define the market the same way you did.

The predominant test is market or pricing power, whether the people you are selling the good or service move to putative competitors in response to pricing/conditions changes you make. If Apple makes their terms more onerous, does it drive purchasers of their app distribution service to the Play Store? If not, they aren't competing in the same market.

Epic Games hasn't purchased any app distribution services from Apple or Google. Epic is the vendor here, not the customer. The money flows from Apple to Epic.

What Apple is doing feels unfair, but making an antitrust case against them is going to be a stretch with current laws.

If I want to buy a game to play on my phone, and there is only one seller, it's a monopoly. Yes, I could buy a new phone, and then buy the game from a different store. But then I'd be buying a different product (Fortnite for Android instead of Fortnite for iPhone).

It depends on how the court decides to define the market. Is the market limited to Fortnite on Apple devices? All games on Apple devices? Fortnite on all devices? All video games everywhere? The laws and precedents provide some general guidance but there's no clear and simple procedure defined for deciding what constitutes the "market" in the context of a particular case. Don't presume to know what a court will decide. In particular the legal concept of a product doesn't work the way you describe.