How does Google fit know that I was working out?

[Android Central Question]

I did not open the app or enter anything manually nor was I wearing a fitness watch. I just went to the gym and cycled for 30mins, and it said that I earned heart points and knew the duration. I kept the phone on the machine display and was watching Netflix. It wasn't on my pocket to detect via the g sensor. I'd like to know how it detected my activity. Is it from location tracking? Did the phone connect to the exercise machine? Its a note9.

 

[pizza_pablo]

How about a smartwatch or fitness band?

There may be a better forum in which to ask.