How does typing.Never enable exhaustive match checking?

Question presented to candidate: "typing.assert_never takes a parameter of type Never. What is Never, and why does annotating that one parameter let a type checker prove a match statement covers every case?"

What a strong answer should cover:

• Never is the bottom type: the type with no values. A function returning Never never returns normally (it raises, exits or loops forever); a parameter of type Never can accept no argument.
• Never was added in Python 3.11 (typing_extensions before); NoReturn is older and checkers treat the two as the same type. At runtime they are distinct special forms that cannot be instantiated or used with isinstance.
• Narrowing: the checker subtracts each handled type from the subject's declared union; once everything is handled, the remaining type is Never and that branch is unreachable.
• assert_never(arg: Never) -> Never turns "the leftover type is not Never" into an argument-type error that names exactly the unhandled type; when the match is exhaustive the call is fine (and mypy does not flag it as unreachable).
• Because a call returning Never ends the path, code after it is unreachable and functions like fail() or sys.exit() satisfy any declared return type.

Code / implementation expected: Yes — a match over a union ending in case _: assert_never(x), and the checker output with and without the missing case.