🔸On iOS 11 (or later):
1. Go to Settings > Control Center and scroll to More Controls (or tap Customize Controls if you have iOS 13 or earlier), then tap the plus button next to Screen Recording.
2. Pull up your Control Center by swiping upwards from the bottom of your screen and tap Start Recording. Then wait for the three-second countdown.
3. Open Control Center, then tap the red Record button. Or tap the red status bar at the top of your screen and tap Stop.

🔸On Android
1. Swipe down twice from the top of the screen to open Quick Settings.
2. Look for Screen Record (or Screen Recorder) — if not visible, tap the ✏️ Edit icon to add it.
3. Tap Screen Record.
4. Tap Start to record (may show a countdown). To stop recording, tap the Stop icon.

🔸On Windows: see here

🔸On Mac: see here