Links to site open browser, not PWA

Hi,
On both Android and iOS devices, if I send a link to my site to someone who already has the PWA installed, it always opens their default browser, and not the web app. Is there any way to change this behaviour? There appears to be, at least on Android devices, a facility for the user to change the behaviour in the settings (Apps > select app >(App info) Set as default, then Go to supported URLs with a choice of In this app, Always ask, or In other app, but whichever option I choose it always opens the default browser. Is there something that needs to be added to the manifest file or is it just not possible? I am assuming that until iOS fully supports PWAs that its not possible but Android? I hope it can be done.