Hi,
Thank you for posting your question to Microsoft Q&A forum.
Below thread describes a way to fully replace the Windows shell with the specific application. Please kindly check if it is what you need.
How to boot directly into an application
Also, we suggest that you could try to use Kiosk mode. A single-app kiosk uses the Assigned Access feature to run a single app above the lock screen. When the kiosk account signs in, the app is launched automatically. The person using the kiosk cannot do anything on the device outside of the kiosk app.
Set up a single-app kiosk on Windows 10/11
Best regards,
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