Believe it or not there are entire companies that are (to simplify, of course) just that: https://www.adjust.com/

Yep, that's what it is.

I thought it was something clever, but it shows as URL in QR Scanner, not an APP, which opens browser after activating it, then user would have to confirm redirect to App Store on empty page

TL;DR: it is HTTP redirect based on User-Agent header

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

Err wut... redirecting based on user agents?

I hate QR codes. I don't trust them.

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 open the original Playstore/Appstore page.

And yes, we used userAgent for identifying the platform.

If anyone is interested, here is the code

    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}`;
        }
    });

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