How does a non-designer understand a chipset's functionality?
All chips, from the very simple to insanely complex, have functional specifications. These describe what the chip does. The IC designer will make a circuit to implement that functional spec, while a validation engineer or test engineer will develop a set of tests to check the implemented chip against the same functional spec.

It's not necessary for a test engineer or validation person to know 'what's inside'. In fact, good practice avoids having these folks know the implementation details, lest any design assumptions creep into their tests.

In some very limited circumstances a test engineer may need to know a low-level detail, such as an I/O pad structure, to test it properly. They may also need to know about a larger block, like an A/D or D/A converter, in order to access its design-for-test capabilities. But otherwise they treat the chip as if it were a black box.