It's not universal. It has become more common, but there is no OS-level enforcement

The one thing Apple has done an amazing job of over the years is providing a solid, clean, common application framework for all of their systems.

iOS doesn't even have a universal back button, every app has their own way of implementing it.

Yeah I agree! In general, a product should be either software or hardware, and companies shouldn't have say on the other one. If you're selling hardware (phone, TV, computer) you have no right to lock the software. If you're selling software (apps, games), you have no right to lock it to a specific hardware.