How to do you make both a Razor Page and C# code framework with ASP.NET Core?

I have written a rather simple ASP.NET Core web app, which I would like to use as a framework for my future web apps.

• How can I get both functionalities in one single NuGet package - that is the C# and the razor pages templates for future projects?
• I already started implementing using the Razor Library sample project in Visual Studio, but I got an error that says that 'RenderBody' does not exist in the current context. But I do want to have this functionality and have a manual override from the main project (the new web app) to overwrite the Razor page contents. And yes, I did add _ViewStart.cshtml with Layout = "_Layout" inside of it.
• How do I make this kind of template? I can't seem to find any tutorials for the whole package.