If someone's signing into a mobile app, quite a good backup option for magic links is to show a QR code containing the current URL, on the callback URL.
Assuming you're using universal app links, if they open it on a device with the app installed, it'll go straight there. If not, it'll show the QR code and the user can just scan that with their default camera app on the device they're trying to use.
Assuming you're using universal app links, if they open it on a device with the app installed, it'll go straight there. If not, it'll show the QR code and the user can just scan that with their default camera app on the device they're trying to use.