LOL

Imagine a room with 50 people, we call that full frame.
Imagine another room only half the size and contain the same amount of people. That is our APS-C

The other room being only half the size, if we compare size for size equality, our full frame room only has 25 persons in it while the APS-C has 50.

If i deliver one pizza to an equal room size basis, then 1 pizza get to be shared with 25 people in the fullroom and 50 people in the APS-C.

That is what happen to light going on a lens. It's the same f-stop. Same pizza. Same amount of light, but it is being shared by a lot more pixel. So the pixels receive a smaller amount of light, same as the small room where each person gets a smaller slice of pizza.

That amount to an exposure equivalent that is more noisy compared to the full frame. Specially in lower light conditions.

How do we get the same quality? More light. So any full frame setting needs to be set 1 stop lower for the APS-C to have the same image quality.
We'll we cant have more light because we take a picture of the same event with 2 different cameras.
So we end up with the APS-C having a "practical" equivalent of a higher f-stop number.