The particle が can attach to a noun to identify it as the subject having an attribute in a sentence.
Having overheard a customer saying Cocoa has a sister complex, Cocoa excitedly tells Chino that it has a good sound to it.
かっこいい (“attractive” or “cool”) is the property attributed to it.
Being told she has to memorize the café menu, Cocoa comments on how the variety of coffee is very difficult.
Discussing a collaboration between cafés, Chino says that for her, a mug is her desired collaboration item.
マグカップ (“mug”) is the thing with the attribute.