Rainbow Six Siege is developed using the AnvilNext 2.0 game engine. This engine is a proprietary technology developed by Ubisoft, the publisher of Rainbow Six Siege.
The AnvilNext 2.0 engine allows for advanced graphics and physics, enabling the game's destructible environments and high level of detail. It also supports Ubisoft's RealBlast Destruction technology that facilitates realistic collision and destruction mechanics in the game.
While this engine isn't available for general public use outside of Ubisoft, similar results can be achieved using publicly available game engines such as Unreal Engine or Unity, both of which support advanced graphics, physics, and environment interactions with suitable scripting and asset creation.