…balance the upper and lower body, left and right, front and back. So a variety, and alternating days which then allows some muscles to recover and supercompensate while you work others.
Remember, it isnt the workout that makes you better—it is the recovery and finally the SUPERCOMPENSATION that follows the workout. That is where the improvement comes.
If exercise alone provided the total benefit, then you should be able to immediately do the 2nd set of 20 pushups [as fast as possible] FASTER than the first set. But, no way..you need recovery and actually recovery and supercompensation before you can do the second set faster. So it will never be faster until you allow an adequate recovery time after the first set. [my analogy doesn’t allow for some help from getting warmed up etc. But if all else is even, the workout alone will not improve your fitness until you have taken a rest of those muscles].