Yes, there are punches and blocks that require upper body strength, but the majority of the focus is on using the legs – the strongest limbs in the body – as the primary “weapon”.
While anyone can get stronger, faster, better by playing a team sport and/or training in many martial arts, Judo specifically involves the full body.  Unlike other martial arts (Taekwondo, for example) Judo uses nearly every muscle in the human body.  As an example, Taekwondo (one of the most popular martial arts in the USA) focuses primarily on kicking.  Yes, there are punches and blocks that require upper body strength, but the majority of the focus is on using the legs – the strongest limbs in the body – as the primary “weapon”.  A Taekwondo student will develop extremely strong legs, but not so much upper body strength.  Judo, as a comparison, uses grip strength, arm strength, back, legs, feet, and, importantly, core.  Kicking a bag, vs picking up a whole person and throwing them six feet through the air requires a vast difference in muscle strength.  Judo utilizes the whole body, so there are few muscles that aren’t challenged by this highly effective martial art.