No problem George. I'll try and be as detailed as possible.
When you create OutSystems flows and logic, these are called Actions. They're represented by the flows you see in Service Studio. But there's another way to implement logic in OutSystems and that's using Extensions. When you install Service Studio, it comes bundled with the Integration Studio:
The Integration Studio allows you to create Extensions, and they're snippets of C# code you can implement and then use as normal OutSystems Actions. This allows you to implement more specific code components for your applications.
After creating an Extension, you can design your Actions and their signatures:
And once this is done, you can edit the corresponding C# code:
This will open your C# IDE and allow you to edit the code - this is where you'd use the C# example your SMS provider has supplied. I use Visual Studio. After finishing your code, you can publish it to your OutSystems environment, and it will be available for use in your applications.
If you've never configured Integration Studio, this is a good primer. If my explanation was a little confusing and you'd like another angle, this course lays it out better than me.