>The Azure Sphere OS is a custom Linux-based microcontroller operating system created by Microsoft to run on an Azure Sphere-certified chip and to connect to the Azure Sphere Security Service

Also

>CBL-Mariner (in which CBL stands for Common Base Linux)[3] is a free and open-source Linux distribution that Microsoft has developed. It is the base container OS for Microsoft Azure services[4][5] and the graphical component of WSL