Look at how android does it: By some heuristic (i.e. visiting the site regularly and interacting with it), Chrome will suggest you install the App (using Chromes UI). Additionally, you can have a custom UI button on your website, that triggers the install (user interaction is required to avoid install spam).

Neither works for Safari/iOS. The only way to help the user, is to show a step-by-step tutorial how he has to navigate his iOS Safari to install. And on top of that, nothing is shared between the installed PWA and the Safari instance - always requiring the user to re-login after install. It is just poor UX.