> From what I understood the "back gesture" is not universal - sometimes you still have to use that Back link on the top?

It's actually universal when the App is built with the native UI components and it's part of Apple's App design guidelines. It doesn't work when the App is built with UI frameworks that don't adhere to this design convention. Sometimes even if the framework by default supports this, the devs can break it in an attempt to be creative but when it doesn't work, it means poor App design.