Q: Does anyone know what heart rate the apple watch uses to determine excersise as opposed to movement?

You seem to assume that activity is either exercise or movement but not both.  That is not the case, as documented in the Apple Watch User Guide:

"The move ring shows how many active calories you've burned.  The exercise ring shows how many minutes of brisk activity you've done."

Start here:

Use Activity on Apple Watch - Apple Support

Again, based on the above article and the "Workout App" article it references, I doubt that heart rate is the only trigger of brisk activity.