Originally extradimensional beings, when their planet was forcibly merged with the Original Universe, they were with it. They remain extremely powerful beings, each with a variety of powers at their command (for example, User Sarge can teleport randomly and User Shroom can grant himself indestructible skin). However, it is implied that their powers are somehow linked to the technology that they house in the User HQ. This technology can range from cloning devices (which Konta was created from) to pocket-dimensional technology (the likes of which were used in the creation of Endless Sea's backpack and Yon's clone's box).
Whether or not the Users' bodies are 'avatars' of who the forum posters (humans) behind BZPB really are, or beings of their own, remains to be seen.