The Umbaco backoffice is a completely seperate Single Page Application that contains no C#. Every interaction is done through the Management API. So technically you could replace the Umbraco backoffice with something else or you could perform all Umbraco actions using the content management API. So the backoffice is a frontend for the C# ‘core’ that communicate with an API. The C# part is completely seperate from the javascript backoffice SPA.
I guess the closest thing to using C# for backoffice extensions is using Blazor. In the end, if it’s a web component, you can use it in the backoffice, it doesn’t matter how to you get to the web component (vue, react, Blazor).