Basically analyze it post test. Helpful if you start the app at the same time you start ramp test or come up with a little marker. For example. “2 mins into ramp I started HRV recording on app”. Would recommend starting at top of minute so you can compare later. You will basically have two files to manually inspect.
And the apps aren’t going to say LT1 or whatever because you’re not measuring lactate. Up to you to find a .75 or .50 or whatever.
TR should record your data as usual and you would just have HR connected to app via the other signal (ant+ or Bluetooth). You are essentially double recording HR, unless for some reason you would not record HR in ride file generated by TR. I see reason to do that. Keep it simple