I originally had the routes marked footways, but some editors had recently started changing a bunch to indoor corridors and remapping multilevel footways on a single level adding connection/elevators exe.. where they shouldn't be. I recently had to remap everything and switched the multilevel routes to corridors to settle the bad indoor routing that was being created. 

I am not opposed to swithcing the main roues tags to highway=footway+indoor=yes but highway=corridor also needs be able to be routed for foot directions. 

I am a frequent visitor at Ohare being my home airport. But I originally uesd the official Ohare maps combined with some of the airline terminal maps provided on their apps, some resent video tours found on youtube  and my personal knowledge to create the indoor maps and routes.   I used indoor editors such as ID-indoor and OsmInEdit while creating the indoor maps. 