According to Apple's Support, If an Apple Watch Series 4 or later detects a hard fall while you're wearing your watch, it taps you on the wrist, sounds an alarm, and displays an alert. You can choose to contact emergency services or dismiss the alert by pressing the Digital Crown, tapping Close in the upper-left corner, or tapping "I'm OK."
If your Apple Watch detects that you're immobile for about a minute, it begins a 30-second countdown, while tapping you on the wrist and sounding an alert. The alert gets louder, so that you or someone nearby can hear it. If you don't want to call emergency services, tap Cancel. When the countdown ends, your Apple Watch automatically contacts emergency services.
Apple Watch cannot detect all falls. The more physically active you are, the more likely you are to trigger fall detection due to high-impact activity that can appear to be a fall.