“The thigh bone’s connected to the knee bone, the knee bone’s connected to the.....” as the saying goes.
And it’s true. The technical term is “a kinematic chain.” Meaning the movement of one part affects or is linked to movements of the other parts.
Thus a problem at the hip affects the knee and possibly the foot, a problem at the ankle affects the knee & in turn the hip, a distorted pelvis reverberates down into the lower extremities, etc.
So if you come in with hip pain, I'm going to also check your pelvis, knees, ankles, and feet. If you have knee pain, I will likewise check your lower extremity above and below your knee, as well as your pelvis.
Unless the whole kinematic chain is addressed and corrected, you're not likely to get lasting relief.