You are incorrect. Android and iOS are quite similar in design, being essentially Unix based. In fact, iOS like macOS has features of a microkernel as well as a monolithic kernel, one of which have anything to do with included apps.Why? (as far as I know)
This is why Android can, presumably, outlive iOS without major kernel revisions.
- Android OS separates core services from the kernel, with services and kernel bound dynamically at runtime. Services and Kernel are upgraded separately. (The Android kernel is just a Linux kernel.)
- IOS is monolithic, and its core services are a part of the kernel. Ergo, upgrading say, Mail or iMessage, requires IOS to be upgraded in toto.
As far as I know.
Apple has chosen to distribute most of their included Apps as part of a (potentially) complete iOS upgrade, but they can easily choose not to, such as with ShortCuts. Also, with incremental upgrade packages, they could upgrade only a single App with an iOS version number change.