9Checks is a web app. You use it through your web browser, but you can add it to your phone's home screen and open it just like an app.
This is called a Progressive Web App (PWA). You don't need to download 9Checks from the App Store or Google Play.
Open 9Checks in your phone's web browser.
Tap the menu button in your browser.
Tap Install app or Add to Home screen.
Tap Install or Add.
Wording can differ slightly between phones and browsers — "Install app", "Add to Home screen", "Install" or "Add" all do the same thing.
Need help? If you're having trouble installing 9Checks, you can keep using it directly from your browser — everything works there too.