Is there a difference between building muscle and building a muscle? For the vast majority of us it’s probably not a thought we entertain. In theory, however, there is; and knowing that difference might provide insight that improves your training.
The insight I speak of is logic that asserts that smaller muscles can be built under less anabolic circumstances than total body mass; and that individual muscles might benefit from an approach different from the rest of the body if training history and potential are different.
An example of this can be in a recent mass-building period that has gone dry. You got bigger using compound movements and you're happy but wanna squeeze out a bit more mass.
Question is whether there's any areas left with any growth potential. Well, considering your new mass was built with compound movements, there might be some individual areas/joint-angles that didn't develop as generously. Therefore, such areas/joint-angles are likely primed for adaptations!
This is a situation where you might focus on building those individual areas/angles with more focused exercises and volume; hence, building a muscle rather than muscle.
Individual areas that are often left with growth potential after most of the body is done, include the biceps, triceps, quads, and hamstrings. Though these areas grew a bit with compound movements, they usually have some adaptational potential left if challenged with focused exercises.
Such focused exercises will likely challenge the muscle-groups at joint-angles they're weaker in; and weak areas are areas with potential (weaker muscle-groups respond to training quicker than stronger ones just like unexposed skin is more sensitive to the sun)!
Additionally, the gorging that characterizes total-body mass-building isn't as necessary when a smaller amount of tissue is being repaired. Again, provided you're not attempting to beat a dead horse with your compound movements!
So understanding the theoretical difference between building muscle and building muscles is important because even after recent total-body growth; additional growth might be redeemable. If there are muscle-groups/joint-angles that are weak, they will be more prone to grow. Trick is to find them and decide which are "buildable" at the time!