I think you'd want your app to hook into either iOS HealthKit or Google Fit. The device becomes responsible for step tracking and general monitoring. That way, you don't have to worry about your app running background processes to track this info. So, your app becomes more of a subscriber to the fitness data captured, and then you can slice and dice that any way you see fit to meet your objectives.

For a proof of concept, my colleague built this widget, which simply displays your step count for the past few days on the screen. To use it, you need a signed Android app and this plugin added to your app. See the instructions on that repository for more info.

Check out https://runkeeper.com/developer/healthgraph/getting-started or https://strava.github.io/api/ for example. You can use the new chart widgets to visualize etc.

If the device in question is a Garmin, then it's not too complex to use their Health API to get at the data you want.

https://developer.garmin.com/health-api/overview/

If you have a Garmin Smart Watch, this would be pretty easy to do.  I think the steps required would vary a lot between device types for Apple watches, smart watches, phones, etc so you might have to ask the user what type of device they are using when they create an account.

I built an integration into the Fitbit APIs: https://dev.fitbit.com/. Which work quite well. It's an extensive REST API allowing to retrieve data and also set up goals.