How would you build a system for running experiments such as A/B tests on an app or website?

To build an A/B testing system, I'd first clearly define the experiment's objective, such as improving conversion rates or user engagement. Then, I'd identify the specific variables to test, like button text or layout elements. The next step involves instrumenting the application or website to randomly assign users to different experiment variations (A, B, etc.) and track key metrics. Finally, I'd implement a robust data collection and analysis pipeline to measure the impact of each variation and determine the winner based on statistical significance.