One small detail that made such a difference in Apple's products was the unique shape that different apps for different platforms had. This is still somewhat a thing, but it seems to be dying off.
Square
The most popular of these shapes is simply the square. These are used for iOS apps. There isn't anything too interesting about this shape, but for a mobile interface, I think it looks nice. It leaves not too much empty space in the home grid.
In the early days of Android, app icons were freeform, so having some consistency was nice. Folders would also retain the same shape as app icons.
Circle
Originally, circles were used for Apple Watch apps. This worked well because the Apple Watch homescreen had apps laid out in a honeycomb grid that could be scrolled in any direction. Now, the Apple Watch offers a list view instead, and the grid only scrolls up and down.
However, when the Apple headset was announced, that also used circle app icons. The main difference is that app icons on the headset used a parallax effect. I know there are only so many shapes, but have you ever considered a honeycomb?
Rectangle
Wide, horizontal rectangles are used for Apple TV apps. This is likely because a rectangle is the shape of a TV screen. Like apps for the Apple headset, Apple TV apps can have a parallax effect, and are required to. Most developers just put their logo on a colored background and that's the parallax.
Pill
The pill shape is used for iMessage apps, kinda. This used to be because of the app carousel that would appear on the keyboard, however in iOS 17 this was removed. Third party apps still use the pill shape, but first party iMessage apps now use a circle. During the iOS 17 beta, some of the first party apps were freeform, which looked nice, but that has been changed.
My problem with this change is that wide pills don't work well in a vertical list. They look really out of place, and feels overall really forced. But then again, Apple is starting to cheap out in software too, so who can be surprised.
Freeform
Finally, macOS apps are freeform, and can have any shape they want, kind of.
With the release of macOS Big Sur, all the app icons were changed to be squares to match the new appearance of the dock. The difference is that macOS app icons can have objects dangling off the side of them. For example, a hammer is used on whatever the macOS IDE is called.
This is cute, but it would of been better if the apps were just freeform anyways, without the square best guidelines. It looks stupid in my opinion.