How do I fix apps not showing up to use as default?

Try one of the following options:

Option 1: Re-register the apps via the installer

Because Windows protects file-type associations under the new user profile, the cleanest way to get Zen, MPV, or VLC back into the main Settings > Apps > Default apps list is to trigger their registration entries again. Download the latest installer for the application and run it. If the installer provides options such as "Install for all users" or "Register file associations," enable the appropriate option. This can recreate the registry capabilities entries that Windows 11 uses when populating the Default apps list.

Option 2: Use the manual "Open with" method

If the application still does not appear in the main Settings list, you can assign it to individual file types through File Explorer. Right-click a file, such as an .mp4 or .html file, select Open with, and then choose Choose another app. Select VLC, MPV, or Zen, enable "Always use this app to open [file extension] files," and click OK. Repeat this for each file type you want associated with the application.

Option 3: Use a third-party open-source tool

A command-line tool SetUserFTA (Set User File Type Associations available at https://setuserfta.com/) or a GUI equivalent Default Programs Editor (available at https://defaultprogramseditor.com/)

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hth

Marcin