What app is going to design to flow nicely with a iPhone Duo but not a iPhone 18? Unless Apple has a 1.4:1 ratio across their entire product line, app designers have to deal with variation.
Also, when you rotate the device, the ratio changes. This is especially common when unfolding a foldable.
Folded and unfolded will both be designed for 2 aspect ratios. 1 for Portrait and 1 for Landscape. Opening the fold and rotating 90 deg will give you the larger version while keeping the aspect ratio
"larger version while keeping the aspect ratio" is of minimal benefit. You still need to present a different layout, because eg. any toolbar is going to be a different fraction of the available area depending on whether the app is on the small outer screen or the large inner screen in portrait orientation. Nobody wants a naive 2x scaling of the phone app on their iPad.
You don't seem to be engaging with the point, which is that the space available for content after subtracting out the space taken up by other UI elements is not going to have the same aspect ratio folded and unfolded. A real UI doesn't actually get this supposed benefit. Only something like a full screen video player might (depends on what kind of keep-out zone you need for the front camera), and maintaining the same aspect ratio folded vs unfolded for video content isn't particularly helpful when the aspect ratio in question isn't popular so you'll have letterboxing regardless.
People keep saying it’s so an app can resize to the full interior screen but none of the marketing photos actually have this. So its hard to say “what a genius masterstroke by Apple.”
Most have the closed screen plus another view. Or the lock screen which is a full photo open and a small section closed.
Im not saying its not possible but it doesn’t seem like that is their intention.
Also, when you rotate the device, the ratio changes. This is especially common when unfolding a foldable.