Have you ever used gmail in your Android or iOS using your browser. While browsing it you get a small popup at the top saying that to view/install the native gmail app available at Google Play or AppStore. How do you do that? Is that feature done using html, css, and JS. Or is it built-in to native app?
By the way, this doesn't happen only for gmail but pretty much all the professional apps with both web apps and native apps.