The basis of designing anything comes from the goal of this product. Such as the goal of an aircraft is to transport people from point A to point B in an easy and efficient way while flying in the air. Then looking on any previous or existing designs which will aid the upcoming new one. After that engineers apply the fundamentals of the science behind building up an aircraft.
As the goal of military aircraft is high maneuverability and high performance, we see that fundamental is high speed which translates to big engines. The engines, big to push enough thrust, are literally the fuselage of the jet fighter and the small slim wing with one or two-seats for pilots.
Usually, the designers start by analyzing the previous designs to see the limits of each design and how to come up with an improved design. Moreover, A rough sketch is produced to give a view of the aircraft will look like from the outer skin. As the designers and engineers agreed on the shape, they start in a CAD drawing or detailed drawing on computers with initial parameters.
After the detailed drawing is done the aerodynamic performance calculation is applied in which the lift and the drag from each part of the aircraft (wing, body, tail) is calculated with high accuracy to manipulate the shape easily to suit the goal. The lift and the drag will help to determine the power train needed for the aircraft, how long it can fly, and the stability calculations. As the final design is agreed on the structural engineers start to seek how to build the structure and materials needed to build the aircraft.
This is a glimpse of how the aircrafts are designed in the early stages. So are you interested to know more?