I agree with the other commenters that this is a pretty simple engineering problem, but I still enjoyed reading about edge cases the author worked to address (they could have taken the easy road with this but tried not to). I forgot that Safari on iPad doesn't specify iPad in user agent headers

For our apps Slyp and SlypBusiness, we had the same QR code for each app open Playstore/Appstore page of the app depending on the phone. Hopefully, we will start rolling out Slyp next week.

Just have a branded link (like https://slyp.app/Slyp or https://slyp.app/SlypBusiness) to the official web page and have the js in that page auto-redirect and open the original Playstore/Appstore page.

And yes, we used userAgent for identifying the platform.

If anyone is interested, here is the code. May not cover all edge cases but works for our usecase. The code was wrung together in a hurry a year ago.

    window.addEventListener('DOMContentLoaded', (event) => {
        let app = 'Slyp'

        var userAgent = navigator.userAgent.toLowerCase();
        if (userAgent.indexOf('android') > -1) {
          window.location.href = `https://play.google.com/store/apps/details?id=com.ndcurve.${app}`;
        } else if (userAgent.indexOf('iphone') > -1 || userAgent.indexOf('ipod') > -1 || userAgent.indexOf('ipad') > -1 || userAgent.indexOf('mac') > -1) {
              if (app === "Slyp")
                window.location.href = 'https://apps.apple.com/app/<app_id>';
              else if (app === "SlypBusiness")
                window.location.href = 'https://apps.apple.com/app/<app_id>';
              else
                window.location.href = 'https://slyp.app';
        } else {
          window.location.href = `https://play.google.com/store/apps/details?id=com.ndcurve.${app}`;
        }
    });

Is this a whole blog post for a simple backend that just routes based on the User-Agent?

Err wut... redirecting based on user agents?

it's just a server side redirect based on the user agent headers