When I apply a force on a body, it will apply a force on me (Newton's third law). But where does this force on me (i.e. equal but opposite force) generate from? Does it generate from within the body, or from somewhere else?
It follows from the principle of conservation of momentum. Think of it as a transfer, like a payment. If you give me 20 dollars, I have 20 dollars more than I had before and you have 20 dollars less because you have transferred money to me. Giving me 20 dollars has an equal and opposite effect on the amount of money you have. It is the same with forces. If you apply a force to me, you transfer momentum to me- my momentum increases and yours decreases by an equal and opposite amount.
For example: I do a force on a broken car to move it. But instead of put my hand directly on it, I hold a spring, and there is another equal spring fixed on the car. So I do the force by contacting both springs.
No matter if the force is enough to move the car or not, the deflections of the springs are the same. So the forces are equal.
We could imagine a different outcome of the experiment without any logical contradiction I believe. So it is an experimental fact.