This is one of Amazon's most-asked problems. Alexa converts numbers to spoken words millions of times per day, and this problem mirrors that exact task.
The core skill is chunk processing. You break the number into groups of digits, handle each group with a helper function, then attach the right scale word (Thousand, Million, Billion). Once you see this decomposition, the problem becomes repetitive sub-tasks rather than a single overwhelming conversion. You'll also get practice handling edge cases like zero, teens, and numbers with internal zeros.