It’s a PWA (Progressive Web App) and while it’s not an actual app, it functions as one without having to open your browser on whatever device you’re using!

You can just go to the FriendRewind download page on Safari (or whatever search engine you use), access the share button, then click “add to home screen”. If your phone shows “open as web app”, turn it on and then press “add”. 

Although I am unsure if it would work on Android, different types of devices (like iMac, MacBook, or iPad) or older iOS devices.