I really miss small phones. Used to be a time when manufacturers were competing to make them as small as possible and Dell’s 5" phone was universally mocked as too big.
Keep is also a Progressive Web App (PWA) which means you should be able to install it from your browser’s menu, without needing a native app and irrespective of your phone’s OS (iOS or Android).
Just go to keep.Google.com and then check if your browser menu has an “Install” option.