first - it's not hard to make. Outside screen size doesn't have to be half of the open screen.
second - main idea is about proportions, if you want to design apps that seamlessly transfer from one screen to another they have to have the same proportions of the sides. This is not a given and depends on the height and width.
If you take iPhone 18 Pro resolution : 2,622 x 1,206 doubling it would make the screen with 2,622 x 2,412 - almost a square screen.
They chose to have the folded phone to have 1.4:1 aspect ratio which is consistent when folded and when unfolded.
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.
The tools can look however the dev wants them to. The utility is for content. It is an aesthetic choice
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.
You're right, it will only look the same for content where the other UI elements disappear
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.
A lot of people will look at this and say. Eh I already have an Ipad and a Imac why do I need a phone with such a large screen.
Their selling point is: Not having to switch from one app to another when Multitasking.