Yeah, it’s tricky and not clearly indicated, but TR automatically pics the “BEST” source for any data if there are more than one of a give type connected.
If you have a smart trainer with power, and a power meter, TR is supposed to pick the power meter for data. You can force it to take the smart trainer by setting the power meter to “Cadence Only” in the Devices menu.
The only way to really KNOW what is being used it the method of checking that I showed above. That can fall short if a person is getting power dropouts. TR will swap behind the scenes at any point the “BEST” one is lost and there is a backup available. Could be confusing for some cases where dropouts are an issue.