There are many elements that go into designing a productive weightlifting workout.
Every exercise has many possible combinations of weight, reps, and time that could be used. People make progress at different rates. Individual muscles recover at different rates. New muscle tissue grows at different rates.
And you need to decide personal preferences like wanting productive workouts as frequently as possible, or wanting to workout efficiently so each workout will yield the maximum benefit.
So, if you want to train scientifically and rationally, a lot of personal factors need to be taken into account simultaneously.