Check out example codes for "redirect users to itunes app store or google play store". It will help you in understanding the concepts better.

Code Example 1

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8" />
<script>
function getMobileOperatingSystem() {
  var userAgent = navigator.userAgent || navigator.vendor || window.opera;

      // Windows Phone must come first because its UA also contains "Android"
    if (/windows phone/i.test(userAgent)) {
        return "Windows Phone";
    }

    if (/android/i.test(userAgent)) {
        return "Android";
    }

    // iOS detection from: http://stackoverflow.com/a/9039885/177710
    if (/iPad|iPhone|iPod/.test(userAgent) && !window.MSStream) {
        return "iOS";
    }

    return "unknown";
}</script>

<script>
function DetectAndServe(){
    let os = getMobileOperatingSystem();
    if (os == "Android") {
        window.location.href = "http://www.Androidexample.com"; 
    } else if (os == "iOS") {
        window.location.href = "http://www.IOSexample.com";
    } else if (os == "Windows Phone") {
        window.location.href = "http://www.WindowsPhoneexample.com";
    } else {
        window.location.href = "http://www.NowherLandexample.com";
    }
}
</script>
</head>
<body onload="DetectAndServe()">
</body>
</html>

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