Ecommerce site backend [closed]

I want to understand how the backend works. Im writing some experimental template site that I built using .net core 3.1 mvc. Now my main question is what counts as a server code, Does the Usual C# code that exist in the .net core project counts as server code and attackers wont be able to see or accsess that code, or do I have to create a new project that will serve as server and will get client requests from the c# files that are in the .net core project. Im Really new with that and I dont know much about it, I just want to undestand how the server/client thing should work and seem.