Hello Sjt and welcome to our forums,
By default, an MSIX is installed/extracted in the %ProgramFiles%\WindowsApps
This is a system location, inaccessible, by default, from Windows Explorer. Only the OS can write in this location when installing your app.
If your application needs to read from a file, it is recommended to write that file in the %appdata% folder, so your application can have access to it.
For more infromation about this, please have a look over our "Install Location"
Hope this helps!