Our unnamed narrator sticks pretty close to Johnny Tremain but keeps a distance that allows for a clear view of him. Check out this passage:
By temperament Johnny was expansive, easily influenced. Although Rab would have been exactly the same if he had been the son of the wealthiest merchant or the poorest tinker in Boston, Johnny would not. When he had been the prize apprentice of Hancock's Wharf, the envy of all the other masters, the principal bread-winner of the Laphams (and he knew it), he had been quite a different boy from the arrogant, shabby young tramp of late summer and early fall. (5.4.1)
Most of the time, the narrator describes what Johnny sees, feels, or knows, but occasionally, the narrator slips out of Johnny's head to tell us something Johnny doesn't know. For example, when Johnny visits Paul Revere in Chapter 2, the narrator tells us, "He did not know all the master silversmiths had an eye on him" (2.1.6). When the war starts, the narrator says,
It was dawn. He was alone in the surgery and still sleeping. But out in Lexington on the Village Green the first shot was fired. One shot and then a volley. And Major Pitcairn was saying, 'Disperse, ye rebels, ye villains, disperse! Why don't ye lay down your arms?'
The war had begun.
It was dawn on the nineteenth of April. But Johnny Tremain still slept. (10.4.32-34)
This narrative approach may indicate two things: the story is about Johnny Tremain, but it isn't about only Johnny Tremain. He is part of larger events.