When you compile from Visual Studio it is obvious: in the Bin
There is however a thing called "GAC" (Global Assembly Cache) which is more for system dll's, read more about it here: Using Assemblies in Microsoft .NET and C#
Normally you won't write to the GAC for your own programs and you can just copy your .dll's and .exe files in one directory for distribution.
To quote Microsoft:
Global Assembly Cache | Microsoft Docs
Quote:You should share assemblies by installing them into the Global Assembly Cache only when you need to