The whole body, every session. Pick this one if you can't decide.
Two sessions take turns. One leads with a squat, the other with a hinge, and both have a push and a pull after it. Run it on 2-3 days. Fewer days just means each lift comes back less often. Set your goal to Build muscle to grow, or Lose fat to lean out. The app paces your lifts to match. New to training? That's fine, every lift starts with a measurement. How it decides explains every rule the app follows.
On four days, Upper/Lower gives each half of the body its own day. If you want strength on the bar more than size, Starting Strength+ runs the same days.
Got a fourth day? Upper/Lower hits each muscle from more angles.
Three days, the whole body each time. The default if you can't decide.