[MirrorField]

Why do their planes always look like our planes?

Convergent Evolution, shared public research and espionage. Not necessarily in this order.

Laws of physics are same to all of us. If certain shape in certain situation gives optimal results, items in that situation tend to look alike whether R&D was done by Boeing or MiG. Likewise, you cannot keep a natural law secret; someone else will eventually get the same idea, especially if he has broad hints and knows something is possible. There is a reason Teller-Ulam design for hydrogen bomb is/was also known as "Andrei Sakharov's Third Idea".

[Loud Mime]

Remember the days when the Russians built their aircraft with blurred metal? Never saw a good photo.