CountLog is a Progressive Web App (PWA), which means that it is built like a website, but runs just like a native app. You can read more about the technical details from Google. For installation purposes, this means that you do not need to go to your app store to download the app.
To install CountLog, simply visit https://app.countlog.xyz.
On mobile, find an "Add to Home Screen" option in your browser's menu.
On desktop (only works with Chrome), open the menu and click "Install CountLog".
You can also choose not to install the app, and only use it in the web browser. For this particular app, there would be very little difference in behavior; the benefit for native installation is user convenience.