How does Citi Bike charge?

Can you explain to me the process behind how Citi Bike charges customers for using their bike-sharing service? Do they have a flat fee per ride, or is it based on time or distance? Are there any membership options that provide discounts or additional benefits? Additionally, are there any hidden fees or surcharges that customers should be aware of? I'm interested in understanding the overall cost structure of using Citi Bike and how it compares to other bike-sharing services.