While recording an exercise, you can lock the buttons by keeping the lower button pressed and toggle Button lock on. Once locked, you cannot perform any action that requires button interaction (create laps, pause/end exercise etc.), but it is possible to change the display views.
To unlock everything, keep the lower button pressed again and toggle Button lock off.
You can customize a lower button shortcut for locking the buttons and the screen with one button press when you are not recording an exercise. Select Button lock under Customize > Bottom shortcut. Then you can lock and unlock the buttons and the screen from the watch face by long pressing the lower button.
When you are not recording an exercise, the screen becomes inactive and dims after one minute of inactivity. To activate the screen, press any button.
The screen also goes to sleep (blank) after a period of inactivity. Any movement turns the screen on again. See Automatic display brightness for information on the screen behavior.