my question is doesn't it take two planes to collide?
In the real world yes, because there is no lag. Over the internet, what you see on your screen can look somewhat different on the other players' screen. Different enough that it may show a collision only on one player's computer, while on the others they narrowly missed each other. What you see is what you get
how is it determined what plane receives damage and collision?
If you have your plane colliding with any object on your computer, you take damage. Same for everyone else.
is it that my internet is faster and sees stuff faster or is it that mine is slower and I am seeing stuff later than others?
No. AH is not played on a server, but on your own client. In a collision you , you both are affected by the combined lag of both player's connections (simplified said)
The higher this combined lag is, the more probable a one sided collision will be.