Robot.java

Robot.java is where everything originates. Here's the WPILib doc page that explains it in more detail.

Key methods you should know about

robotInit() is called when the robot code starts
robotPeriodic() runs periodically regardless of Operational Mode (see note below)

Operational Mode refers to whether the robot is in Teleoperated (Teleop), Autonomous, Disabled, or Test mode.

teleopInit() is called when Teleop is Enabled
teleopPeriodic() is called periodically when Teleop is Enabled
autonomousInit() is called when Autonomous is Enabled
autonomousPeriodic() is called periodically when Autonomous is Enabled
testInit() is called when Test is Enabled
testPeriodic() is called periodically when Test is Enabled

And finally,

disabledInit() is called when the robot is Disabled
disabledPeriodic() is called periodically when the robot is Disabled

A quick excerpt is included below:

import edu.wpi.first.wpilibj.TimedRobot;

public class Robot extends TimedRobot {

  @Override
  public void robotInit() {}

  @Override
  public void robotPeriodic() {}

  @Override
  public void autonomousInit() {}

  @Override
  public void autonomousPeriodic() {}

  @Override
  public void teleopInit() {}

  @Override
  public void teleopPeriodic() {}

  @Override
  public void disabledInit() {}

  @Override
  public void disabledPeriodic() {}

  @Override
  public void testInit() {}

  @Override
  public void testPeriodic() {}

  @Override
  public void simulationInit() {}

  @Override
  public void simulationPeriodic() {}

}

Bonus: Main.java

If you take a look at Main.java (copied below), do you notice anything?

package org.aceshigh176.frc2023;

import edu.wpi.first.wpilibj.RobotBase;

/**
 * Do NOT add any static variables to this class, or any initialization at all.
 * Unless you know what you are doing, do not modify this file except to
 * change the parameter class to the startRobot call.
 */
public final class Main {
    private Main() {
    }

    /**
     * Main initialization function. Do not perform any initialization here.
     *
     * <p>If you change your main robot class, change the parameter type.
     */
    public static void main(String... args) {
        RobotBase.startRobot(Robot::new);
    }
}

That's right, this is what calls the code in Robot.java

Robot.java

Bonus: Main.java​

Bonus: Main.java