How does an iPhone collect data about my resting energy?

Apple doesn't tell us how their phones throw the data we've entered, and the data from their sensors, etc., into a pot to come up with estimates of what we've been doing. But some things seem likely.

• You may have told your iPhone your weight, which would be one of the major factors influencing the number of calories you burn while at rest.
• The iPhone may be guessing when you are "active" and when you are "at rest" based on data from acceleration sensors and/or location services, combined with a bit of processing.
• If you have an Apple Watch and are using it for health-related features, maybe the watch is gathering data to be sent back to the phone. ([EDIT]: But I see that you're not using an Apple Watch.)

I don’t know. You’d have to ask them.