To set an AppImage application as the default app, you need to create a desktop entry for the AppImage.
Step 1: Create a desktop entry for AppImage using Gear Lever app or similar tool.
Step 2: Set the default app through the file manager by right-clicking on the file and selecting 'Open With' option, then selecting the AppImage application and enabling 'Always use for this file type' toggle button.
Alternatively, you can change the default apps for categories like browser, music player, etc. through system settings.