Who really did the SEPTEMEBER 11, 2001 (9/11) ATTACKS and why some people still not convinced it's terrorists?

The argument is still stands after all these years; it was the terrorists Al-Qaeda who attacked the U.S. that day, which later admitted by its leader Osama Bin Laden but the odd thing was that he admitted that some months or years later, and that leads to some people this whole thing was a U.S. government cover up / conspiracy via seemingly legit arguments like those found in the video "Loose Change", and with the evidences that during the 1960s U.S. government cover up "Operation Northwoods", and the "where's the plane that hit Pentagon?" and "where are the other planes?", so which is it?