ArcGIS Instant Apps | Create Web Apps to Share Your Maps if you don't have arcgis online
Download instant app and deploy to web server - Esri Community
read the first answer
We used to provide the apps on GitHub but then we made some optimizations to share resources between apps and it became time consuming to make the necessary changes to ensure the apps would run as standalone apps so we no longer share the code publicly .
Which app are you looking to download? I'm also curious why you want to download the app? Is it to host it on your own web server? Are there changes you want to make that are not possible in the out of the box app? The answers to these questions will help us prioritize updates made to the apps in the future.