Net Calories = Basal + Active - Consumed
We get your basal calories from Apple Health, which calculates it from your age, sex, and weight. So, this is just an estimate.
Your active calories are based on activities recorded by Apple Health or in other apps, whether it's just the step count recorded by your phone or any other activities recorded by a wearable device or through manual input.
Your consumed calories are usually coming from manual inputs using a 3rd party app/device that is synced to Apple Health.