DLC For Steam/Standalone

Steam
If I release a game on steam and I want to add an expansion/DlC how do I make it appear in the origianl game, and so the DLC adds new levels/scenes etc.

Standalone
If I release a game on my website or what ever and I want to make a dlc for it as a ZiP or what ever how do I make it work?

AssetBundles

is one of the better ways