Don''t worry. I actually haven't been using MVC or Razor all that long but I've been using ASP.NET a fairly long time. Razor hasn't even existed until recently :) ...so I'm still constantly learning things.
You've just got to remember Razor is no different to PHP but with a slightly cooler syntax. It's server code, it lives in .cshtml files - that's
mixed C# and HTML - so it gets parsed and executed at the server by the runtime.
So your browser can't understand cshtml and neither can tinymce, it's not part of any widely-used spec, it's just an arbitrary syntax invented by some really awesome developers.
However... there is a system of filters in Orchard that enable scenarios like, for instance, expanding out BBCode from the body and turning it into html.
So it would be technically possible, perhaps using the .NET dynamic runtime (DLR), to parse and execute Razor code in the body during render.
But I wouldn't particularly recommend attempting that, it'd be pretty complicated, and could be extremely dangerous unless you're very careful and it's only you who's going to be editing the content. If you let someone input artitrary code to run
on the server you could create all kinds of problems.
If you want to do this kind of customisation, you should be doing it in your Theme where you can write whatever cshtml code you like. The other option is to create a ContentPart and attach it to your Page content type, you can then write cshtml display views
for the part.