Samsung
Use the Stopwatch in the Clock App auf Samsung
Samsung does not offer a separate stopwatch app. The function is built into the standard Clock app alongside alarms and the timer.
Open the app and switch tabs to reach it: Tap on Clock ➔ Tap on Stopwatch. From here you control the session with four actions.
Start the count with Tap on Start. While it runs, Tap on Lap records a split without stopping the total time, which is useful for interval training or comparing sections of a run.
When the activity ends, use Tap on Stop to freeze the count. Tap on Reset clears the display and lap list for the next session.
Why it keeps counting with the screen off
The stopwatch continues running in idle mode, even after the display turns off. This is intentional: locking the phone or letting the screen dim does not pause the timer, so you do not lose progress just because you put the phone in your pocket.
If you need the time visible without unlocking, check the always-on display or lock screen widgets, since the stopwatch itself has no persistent notification by default.