The .cs file is the code behind file for every class.
Code-behind files are files that are pre-compiled modules written in any of the Microsoft .NET runtime compliant languages.
These are the dll that are compiled and stored on the server (mostly in the app-code directory).
The code behind file is nothing but a partial class and was introduced in .Net 2.0.
To read more about code behind versus code inline have a look at
Code Behind vs. Code Inline[
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