How to implement DLC
Since you tagged Unity, I assume you're wanting to build something in there even though you didn't mention it in your post.
Unity has a feature it calls an AssetBundle.
The details can be found in the link, but basically you write your initial program so that it can load the asset bundle.
On your own you will need to create some descriptive file or process that has a list of what is in the bundle and how to use it. Then you need to dynamically load all the content and scripts into your game. Once loaded, they are regular GameObject or Texture2D whatever you stored in them.
The exact details are going to depend on your project. You might build an asset bundle of a bunch of high resolution textures and a list of which models they go with. Or you might build an asset bundle with some new game objects, and a list of places in your game they should be spawned at. You might add them to an AssetDatabase that your code searches for lists of assets.
Exactly how you pull them out is something you need to plan for in your design. Also, since people have done it before, DLC is something you need to plan and experiment with BEFORE launching the initial project. You cannot really back-fit DLC into a shipped application. Most games with DLC have several DLC packages at launch as these were the packages used to test and validate that the DLC system is working properly.
