I think Marion has it as close to right as possible from a photo. That large pipe is likely the original "main stack" required passive vent that every house must have. It should extend through and above the roof. The moved a wall, so now the stack is in the middle of the floor rather than in the wall behind the water closet as it would be 90+% of the time. That is a serious safety hazard as sewer/septic gases are highly dangerous. C6 as is.Photo below. Toilet and partially installed sink but what the heck is that thing in the corner?