Increasing calories help build muscle. Generally it’s easier for people to increase their caloric intake by increasing carbs since they are less satiating than protein. That’s what I was getting at.

But to your question the role of carbs in building muscle is mainly by providing the energy necessary for your body to create the new tissue. Spikes in insulin resulting from eating carbs also aids the muscle growth process, which is why a lot of old school body builders used to inject insulin.