The way I see it, it kinda is a conspiracy. But it's not as extravagant or complicated as you might think.
The US provokes Iraq with embargoes starving half a million people. Naturally, slightly pissed off and confused militias originally trained by the US want to get back at them. The US intelligence services that trained them in the first place predict everything they're going to do and know when the attack is going to happen. **** Cheney even knew the exact day, having arrived at the NORAD command centre to take command- a completely unprecedented event before that day- hours before there was any sign of a crisis.
Yeah, they knew it was going to happen, yeah they caused it, yeah they wanted it. But it was mostly done indirectly, no swapping around passengers or remote controlled planes. Whether the towers were brought down by demolitions is extremely hard to determine and mostly irrelevant. The 911 investigation is probably the weakest cover-up since Watergate and the blatant bull**** it spouts proves time and time again that there is something to cover up.
That's about all there is to it, really.