It's... it's basically all products. Virtually any product on iOS would function and perform and integrate better if implemented by Apple. It's just a question of at what point it gets popular enough that they want to bother doing so.
> The same line of reasoning applies to any service
iOS is a special case because of how locked-down it is. Not only can't third-parties do things like draw over other parts of the UI or add items to system UIs, they can't even (truly) run in the background.
Of course, on Android, apps can do all of these things and that's part of why it's a security nightmare. Which is why as I said, there's no clear answer to the problem. But it is a problem.
I looked through every app I have installed on my iPhone and I only found a couple that would fall into that category. I have a shopping list app which Apple could theoretically implement as a feature if it wanted to (but not fully, as this particular shopping list app allows me to share lists with android users), and Authy.
The rest of the apps I have installed are just interfaces for 3rd party services. Apple doesn’t have the power to subvert the value of those services any more than any other service provider does (Apple has a maps app, I still use google maps).
I guess Apple could also make a Sudoku app if it really wanted to, but I’d consider that pretty unlikely.
I think there’s a strong argument for saying something like Spotify vs Apple Music is anti-competitive, but for rather different reasons.
Unless the answer to that question is a clear "yes" in every case, this behaviour is de facto - and possibly de jure - on the wrong side of anti-monopoly law.
The problem is that iOS and MacOS are not specifically operating systems. iOS especially is an OS bundled with a curated non-trivial application suite.
That suite puts Apple into direct competition with iOS developers. And of course Apple has an impossibly huge monopolistic advantage in that space.
The ethical and above-board option would be to buy the rights to these apps and/or acquihire the devs who make them.
Apple has done that in the past, but seems less and less inclined to operate like that now.
It is perhaps a good reason to think twice before making an iOS app though. But as a consumer I prefer it to the opposite approach, which I’d consider a company like Atlassian to have. If you ask Atlassian when they’re going to implement a rather basic and highly demanded feature, the answer tends to be “never, buy it on the market place”.