The reason why American Airlines 747s appeared in the Airport films AND one got sold to NASA for use as 905 was because the airline overbought the aircraft. American back then was primarily a domestic route airline back then (before they snapped up international routes in the airline de-regulation period) and was utilizing the 747s on trans-continental flights between Los Angeles, New York and I believe Washington DC as well. They believed that passenger numbers would be up enough to fill the planes, but the higher numbers didn't materialize and the energy crisis also didn't help matters. One thing they did was to put in a Piano bar in some of the planes, where a pianist would play songs on request for first class passengers on the red eye flights (taking out about three or four rows of seats in the process). But ultimately they ended up parking a few of those planes and so one 747-100 became available for NASA to acquire for use as the space shuttle transport in the early 1970s (after taking part in wingtip vortex experiments for NASA). A few of the other 747s in their fleet ended up getting either sold or leased to other airlines.
So when the Airport 75 and 77 disaster films were filmed, American Airlines 747s were available for repaints. The funny thing is if you watch the 747 in Airport 77, ALL the shots showing an actual 747 in the film with the Stevens colors are of it on the ground. All the aerial scenes (and underwater ones) are of a model or stock footage. Those particular 747s were built I believe in California, although I can't recall for sure if Universal Studios had their own effects house (Universal Heartland) by that point or if they had contracted out to someone else.
Airport 75 made use of an actual 747 with the altered AA livery and a bit of a "makeup job" to make it appear as though it collided with a private airplane (with Clay Lacy's lear jets shooting the air to air footage). That particular 747 was used through the early 2000s as a cargo hauler for UPS after it ended its days as a passenger airliner. I think it is retired from service now, as are most of the 747-100s world wide.
As for the Moonraker connection, the aircraft in the film was essentially NASA 905, complete with the same tail number, but with a Drax logo in place of the NASA one. The real NASA 905 kept its American Airlines colors until about 1984 when it was repainted in the blue and white livery in preparation for taking Enterprise to the Paris Air show that year.