Applications don't show in Volume Mixer
This functionality was removed from Windows 10.
You can use an application called EarTrumpet instead:
Getting Volume Mixer for Modern Windows App
The volume mixer for desktop mode limits the all access pass to only
the desktop applications. If you wish to get similar functionality for
the Modern Apps, you will have to install EarTrumpet application on
your computer. Simply download the .exe file on your computer and run
the setup file.
After the application is installed, you will see a trumpet icon in the
system tray, from where you can control the sound of individual
applications for the modern apps. Also, as this is an individual
application, you can use it in parallel to the desktop volume mixer.
The app is very useful in instances where you are playing a modern
game on Windows 10 and at the same time listening to some songs on the
Edge browser. You can lower the volume of the game and enjoy the song
at the same time. This is just one of the many scenarios where it can
prove to be useful.
Source Selectively Control Volume of Modern and Desktop Apps on Windows 10 Simultaneously