Please refer to the following articles listed below for the iOS PWA functionality
With iOS 11.3, Apple has silently added support for the basic set of
new technologies behind the idea of “Progressive Web Apps” (PWAs).
It’s time to see how they work, what are their abilities and
challenges, and what do you need to know if you already have a
published PWA.
https://medium.com/@firt/progressive-web-apps-on-ios-are-here-d00430dee3a7
PWA on iOS isn't automatically available like on Android, there are some additional settings you will have to change to allow for the popup to show on Safari iOS.
Additionally from the following stackoverflow Link, there is a link to making it available for iOS without the user having to change their settings.