Moreover, I advise that the iPhone software platform must be opened
Apple: Update on apps distributed in the European Union
Apple is sharing changes to iOS, Safari, and the App Store, impacting developers’ apps in the European Union (EU) to comply with the Digital Markets Act (DMA). These changes create new options for developers, including how they can distribute apps on iOS, process payments, use web browser engines in iOS apps, request interoperability with iPhone and iOS hardware and software features, access data and analytics about their apps, and transfer App Store user data.
There really is a lot to cover, and you could spend a lot of time trying to summarize it and still miss things. Here are some thoughts in no particular order.
This is part actually honestly well-meaning, part malicious compliance, part intricate machinery. Many components have all three parts present.
This has been a long time coming and I'm sure a lot of people have been working very hard for a long time to make these things happen. I appreciate their hard work; particularly I know that untangling assumptions built up over the past 15 years does not come without headaches. If Apple leadership had just designed a less ass-backwards platform, fewer changes, maybe even none, may have been required.
This is not sideloading in any commonly used term. Particularly with the financial requirements and the Core Technology Fee, it is devolved monopoly. A third-party marketplace is required to have significant amounts of cash on hand, and driven to be able to recoup €0.50 per first annual install of its own marketstore app (which do not get the one million "grace" window).
On the other hand, there is a sliver of a legitimate point that for aspiring actual marketplaces, being able to provide all the things that such a marketplace should provide. But marketplaces are the answer to a not-locked-down App Store. Sideloading is the answer to a not-locked-down Ad Hoc Distribution; this quadrant is as missing now as it ever was.
App Notarization is required and includes some form of abridged manual App Review where a bare selection of guidelines, relatively speaking, are applied. (It is possible that this lighter review process is a variant of Testflight pre-release scanning.) Following notarization, the signed app is delivered to the marketplace backend directly.
Apple is digging in on its world view that no developer can ever be trusted, even directly by a user. Shipping a bug fix update straight to users, without having someone on Apple QA it, is not possible.
It is not ideal. It is a start. Even when the monopolist is trying to fight back, it is better than the monopoly.