As far as I’m concerned, the .net framework is related to the web front end cluster within the Power BI architecture:
The WFE cluster provides the user's browser with the initial HTML page contents on-site load and manages the initial connection and authentication process for Power BI, using Azure Active Directory (Azure AD) to authenticate clients and provide tokens for subsequent client connections to the Power BI back-end service.
A WFE cluster consists of an ASP.NET website running in the Azure App Service Environment. When users attempt to connect to the Power BI service, the client's DNS service may communicate with the Azure Traffic Manager to find the most appropriate (usually nearest) datacenter with a Power BI deployment.
For more info, please refer to the Power BI security whiter paper: