macOS Mojave is the final version of macOS to be compatible with legacy QuickTime software, including QuickTime Player 7 and apps that use the QuickTime 7 framework. If you have a third-party media app that no longer works with later versions of macOS, check with the app developer for an updated version of your app.
For common media operations such as media playback, media inspection, and basic editing, Apple recommends the latest version of QuickTime Player (version 10), which is included with macOS.
Microsoft Windows versions introduced in 2009 and later support key media formats, such as H.264 and AAC, that QuickTime 7 for Windows enabled. If you no longer need QuickTime 7 on your PC, learn how to uninstall QuickTime 7 for Windows.
If you're still using QuickTime Player 7 and need help, you can find documentation in the Help menu of QuickTime Player 7. You might also find answers from other Apple users in Apple Support Communities.