Hello, Welcome to Microsoft Q&A,
"Is there a way to read/write a file (txt) stored in internal storage in Unity?"
We can use Unity API Application.persistentDataPath to store data or using the UWP API Windows.Storage to manage files.
"Is there a way to access the local storage where the Unity program is installed via USB?"
If the "Unity program" means the MR app built from Unity, there is not any support to deploy to an external USB drive and they most go to the device main storage when deploying to the actual device.
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