I worked extensively with Pluton when I was employed on Azure Sphere (an IoT platform marketed as highly secure and composed of a linux-based OS, ARM SoC, and cloud service). I might be able to answer questions about this. Here’s a blog by the engineer lead on Azure Sphere that discusses Pluton: https://azure.microsoft.com/en-us/blog/anatomy-of-a-secured-... Disclaimer: I still work at MSFT but in a different org.
2. If the latter, "Every piece of software on an Azure Sphere device must be signed by Microsoft." what does the OS interface look like?