Muscle-building does not occur during exercise. Rather, exercise tears down muscles, which then rebuild and adapt during the post-exercise rest and recovery period. That is why, typically, specific muscle groups should not be worked on consecutive days. They should be allowed to recover for at least 48 hours, though 72 hours is generally best. The link below explains the scientific process.

https://www.unm.edu/~lkravitz/Article%20folder/recoveryUNM.html