Hi rsirico,
Welcome to the Apple Support Communities!
I understand you have some questions about how your iPhone and your Apple Watch share information with each other. These apps are designed to stay in sync with the Health app, so there should be nothing to do on your part if your Apple Watch and iPhone are paired. Also, if you forget to start a workout, you can manually add data to the Health app. I'll include a few resources here for you about these topics. I've highlighted some key information here for you. Click on the links for complete details.
Use the Activity app on your Apple Watch
Use the Health app on your iPhone or iPod touch
If Health doesn’t track steps or other information from your iPhone or another device, tap the Sources tab and tap your device or accessory under Devices. Then tap Privacy Settings > Motion & Fitness and make sure that Fitness Tracking is turned on.
Manage Health data on your iPhone, iPod touch, or Apple Watch
The Health app takes similar data from different sources, like steps recorded from your iPhone and steps recorded from your fitness band, and organizes it based on where it comes from. By default, Health prioritizes data in this order:
- Health data that you enter manually.
- Data from your iPhone, iPod touch, and Apple Watch.
- Data from apps and Bluetooth devices.
When you add a new data source, like a step counter app, it appears above all other apps and devices that contribute data for steps in Health. You can change the order of sources at any time.
Take care.