How Apple is assuring that apps compiled against previous SDK's are working under latest iOS releases?

The hallmark of any robust OS is how well it tolerates and survives over time, the details of which are buried in various SDKs and APIs, some public, most private, I suspect.

Private can also mean the 'blackbox' stuff that Apple keeps out of the public's and competitor's view.

We're left to not only imagine the details, but to also admire the hard work, testing and coordination across hardware and software teams, witnessed by the stunningly proficient results that are clearly behind such efforts.

I'd not expect Apple would survive very long if the 7 secret spices and herbs were to become known to the world.

Frameworks, BTW, are compiled/baked into an OS, morphed into the binary, vs. being 'included'.