Mario Builder 64 is a fan-made concept, with no official history. Its gameplay would presumably revolve around a level editor or sandbox mode that allows players to create and share their own Mario 64 levels. This would require advanced programming and likely builds upon the original game's engine.  The creators would be independent developers or modders skilled in game development tools and potentially ROM hacking.  The origin is a creative spin on the Super Mario 64 concept.

Mario Builder 64

Mario 64 DD Episode 1