OnSong is an iOS and iPadOS universal app available for iPad, iPhone, and iPod touch devices. These apps are only available for download from the Apple App Store.
To install OnSong, search for "OnSong" on the App Store app on your device. Choose your version of OnSong to view details about the app. If you haven't purchased OnSong, a button with the price will appear. Tap on this button to complete the purchase and download and install the app.
If you've already purchased OnSong on your AppleID, the button should appear as a download icon. If not, it is likely because you are not currently signed into the App Store. In that case, the App Store does not reveal information like the apps that you have purchased. You can install OnSong by viewing the Purchases list under your Account profile. You can also sign into the App Store in this process or by purchasing a free app to view the appropriate download link in the OnSong App Store page.
Apps are purchased and applied to your AppleID. This means that you can install the app on any device that you own. However, if you have purchased add-ons, you may need to Restore Past Purchases so that these purchases can be unlocked by OnSong.