Build a web app

I am interested in making a web application that is quite similar to an already existing one: https://clipr.xyz The whole purpose of the application is to download clips from twitch onto your mobile phone, specifically ios and android. The website that I linked is catered for desktops primarily and you are unable to save the video to iPhone gallery. The web application we develop must be able to download the clip in all available resolutions to the mobile device.