Mechanical Engineering Major Question

Can i build fighter jets, helicopters, for the USA or engines for these things with a degree in mechanical engineering. thanks

You can work for a company that does. They are Lockheed Martin, Northrop Grumman, and Pratt & Whitney (Engines) for starters. You do not have to be in the army to work for any of these companies. Security clearances may be necessary for much of the work, but again, no military requirement exists.

The folks that build them do not typically have engineering degrees; they're just assembly technicians. But you cannot just do it on your own.........

No
only by doing mech. Engg. U can't build a jet plane for that u need special arms&weapon mastery which is available only in the army of any country