How to make a game communicate and render into UE4

Development Rendering
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Hi, I have a question:
I have the source from a 2004 old game, DX9 and C++ and I want that game to communicate with Unreal Engine 4.
Somebody did that, “its something runtime to communicate between the game and ue4 and render on ue4 without any models & textures etc”
Video: lalaker1 Development
How is that possible ? Thanks.