https://www.theverge.com/2018/6/4/17424638/google-photos-progressive-web-app-android-chrome

Google Photos is the latest Google service to get a Progressive Web App version. It’s following in the footsteps of Google Maps Go, which got its own lightweight web app last December, via Android Police.

To set up the Photos Progressive Web App, go to the Google Photos site where you can now add it to your home screen on Android or desktop (assuming you're using Chrome, of course).