Design a URL Shortener

Background: A URL shortener takes a long URL and converts it into a shorter, unique URL that redirects to the original URL. This process makes sharing URLs easier, especially on platforms with character limits.
Problem Statement: Design a URL shortening service with the following functionalities: shorten(url: String): String to shorten a URL and retrieve(shortened: String): String to retrieve the original URL. Focus on scalability, storage, and handle issues like collisions.
Constraints: The service should support up to billions of URLs and provide low latency.