The engines are able to throttle but they run all of them at 100% throttle for most of the second stage's first burn, I expect. For efficiency. The telemetry showed all of them firing at the same time. The vacuum engines will be used by themselves for on orbit maneuvers and the sea level engines will be used by themselves for landing, but for the first burn to reach orbit they need all the thrust they can get.
Yes, on the first stage.
Oh! Second stage. Misread.
No, second stage has 3 vacuum engines and 3 atmospheric engines, so they'll have to be able to throttle for the cutover.
The engines are able to throttle but they run all of them at 100% throttle for most of the second stage's first burn, I expect. For efficiency. The telemetry showed all of them firing at the same time. The vacuum engines will be used by themselves for on orbit maneuvers and the sea level engines will be used by themselves for landing, but for the first burn to reach orbit they need all the thrust they can get.