Use Apple Watch with iPhone
Pair Apple Watch with iPhone
On your iPhone, tap the Apple Watch app , then follow the onscreen instructions.
See Set up and pair your Apple Watch with iPhone in the Apple Watch User Guide. (Apple Watch sold separately.)
Unlock iPhone with Apple Watch
When you’re wearing your Apple Watch (Series 3 and later), you can use it to securely unlock your iPhone (models with Face ID) when you’re wearing a face mask (iOS 14.5 or later and watchOS 7.4 or later required).
To allow Apple Watch to unlock your iPhone, do the following:
Go to Settings
> Face ID & Passcode.
Scroll down, then turn on Apple Watch (below Unlock with Apple Watch).
If you have more than one watch, turn on the setting for each one.
To unlock your iPhone while you’re wearing your Apple Watch and a face mask, raise iPhone or tap its screen to wake it, then glance at your iPhone.
Note: To unlock your iPhone, your Apple Watch must have a passcode, be unlocked and on your wrist, and be close to your iPhone.
Work out with Apple Fitness+
Used in conjunction with Apple Watch, Apple Fitness+ is a service that lets you choose from a catalog of workouts led by expert trainers. While you follow a workout on the iPhone screen, in-session metrics like heart rate and calories burned (which are captured by your watch) appear on your iPhone screen. (iOS 14.3 or later and watchOS 7.2 or later required.)

See All about Apple Fitness+ in the Apple Watch User Guide.
Collect health and fitness data from Apple Watch
Apple Watch can send data about your health and fitness to iPhone for you to view in Health. Apple Watch can also send notifications about high heart rates, low heart rates, loud environmental sounds, and more to your iPhone.
See Track important health information in the Apple Watch User Guide.