What movable surfaces are used by the falling booster to steer through the atmosphere?
Answer
Grid fins
The booster uses four deployable grid fins located near the top of the stage to steer by differentially angling them to adjust the center of pressure.

#Videos
How SpaceX Rockets Land Back on Earth - YouTube
How Space X Crew 10 Transfer & Return to Earth Works? - YouTube
SpaceX Makes History Catching Rocket on Landing - YouTube
Related Questions
Which SpaceX rocket is primarily associated with this reusable booster landing capability?Approximately how long after liftoff does Main Engine Cut Off (MECO) typically occur for the first stage?What is the name given to the initial engine burn performed by the first stage shortly after separation to redirect its trajectory?What is the primary function of the re-entry burn during the descent phase?How many Merlin engines are typically reignited for the final landing burn?What movable surfaces are used by the falling booster to steer through the atmosphere?What material are the four deployable grid fins made from?Why is landing on an autonomous floating platform often preferred for high-energy orbit missions?How does the Crew Dragon capsule conclude its journey back to Earth?At approximately what altitude does the critical final landing burn commence?