"Default apps" refers to the pre-installed applications that come with your electronic device, such as a smartphone or computer. They are the programs set to automatically open specific files or perform certain functions when you use your device.
Whenever you set default apps for certain activities, these settings are lost once you reboot the phone and you end up setting them up again. (patchworkoftips.com)
Specific default apps on the home screen have shortcuts that can be accessed this way. (pocket-lint.com)
Just go to the Settings app and tap on manage apps section, and then further tap on default app setting. (phoneradar.com)