Picture drawing lines in 3d space in something a little more sophisticated than SketchUp, and then having the drone follow those paths. What's tricky about that? Indoors you have no weather, and could use a beacon system to pinpoint location.
Beacon systems can be used (but might not work well in metal enclosures like in the video).
Just accurate trajectory following is very difficult in itself depending on the speed/accel/turning.
The cars in the video were moving and so the drone had to be time synced. Same with the stuff inside the factory.
They built a factory using a high degree of automation in a 3D space - i.e. they have a significant staff of automation engineers.
Intel has been doing tight formation drone light shows outdoors (in weather) since at least 2014, if not earlier. Positioning drones precisely enough (i.e. within N centimeters) is therefore objectively possible if you have the right feedback system (and as a sibling comment points out, IR sensors could work for that). Offload all the processing to a computer rather than the drone, and it isn't an insurpassable problem.
A small team of engineers for a few months is <$1m, which for a marketing campaign this splashy is nothing. Granted, per one of the sibling comments they did use an FPV pilot for at least some of the shots, but there's nothing stopping them from having automated other parts of it.
I don’t have experience using it in volumes as large as the video, but in moderate sized lab space it works very well, and at high frame rates.
The trick is to invert the problem and have the active sensors off-board and the robot is the (reflected energy) beacon.