What is a continuation (withCheckedContinuation) and why?
The essence
A continuation is the bridge from the callback world into async/await. withCheckedContinuation / withCheckedThrowingContinuation suspend the current task, hand you a CheckedContinuation object, and the task resumes when you call resume(returning:) or resume(throwing:). The contract: exactly one resume per continuation. This is how you wrap completion-handler APIs, single-answer delegates, CLLocationManager.requestLocation, SKPaymentQueue and the like.
The rest of this answer, with code and interview follow-ups, is in the DevPrep app.
Read the full answer in DevPrep