They had knives as well as box cutters. Anyone who's ever used a box cutter knows it can slice a man's throat quite easily.
They did not fly the planes "like professionals." They flew the planes poorly, for a very brief period.
American Airlines Flight 11 was hijacked at 08:14 and hit the WTC at 08:46, 32 minutes later. United Airlines Flight 175 was hijacked at 08:44 and hit the WTC at 09:03, 19 minutes later. American Airlines Flight 77 was hijacked at 08:53 and hit the Pentagon at 09:37, 44 minutes later. United Airlines Flight 93 was hijacked at 09:28 and hit the ground at 10:03, 35 minutes later.
They had sufficient training to fly the planes in the required manner, for the necessary duration.
Their targets were absolutely massive. No "pinpoint accuracy" was required, nor was it involved.
Large amounts of wreckage from all four planes were recovered from all three sites. Bodies of passengers were also identified.
Three buildings fell down because two were hit by planes and one was hit by burning debris, causing it to catch fire and collapse.
Glad I could help.