How are aircraft filmed in sky?
Two words: chase planes.

A few more words: The easiest way to film an airplane in flight is with another airplane in flight. Here's one article on how that's done, including images captured from both sides; the chase plane (a Learjet 25B) getting the shots of the 787 Dreamliner, and crew aboard the Dreamliner getting a few snapshots of the chase plane.




Early shots taken from chase planes often used simple handheld cameras; a two-seater would normally be used with the pilot in front and the photographer in back. This is still a totally viable way to do it, but more modern professional shoots utilize aircraft specifically modified for the task by adding internal or external camera mounts, optical-quality viewports, etc. Here's a modified T-33 used by Boeing specifically for air-to-air photography, including some of the initial publicity shots of the 787 in flight; notice the camera mount between the seats that can be used by the GIB (he's using a handheld in this photo:



For lower-speed shots, you can also use a helicopter. Many of these such as news choppers have remote-control cameras on gyro-stabilized mounts, so the vibration of the airframe is minimized in the shot even at high zoom:



These same camera systems can be used for air-to-air shots as well, however the maximum airspeed of the helicopter becomes a limiting factor. Still great for publicity shots of small prop planes, or takeoffs/landings.