Which iPhone is earned in study?

If your are talking about Apple’s study then the article link above explains you must have at least an Apple Watch series 3 or later and watchOS 7 or later. You do not ‘earn’ a watch, there is no free Apple Watch offered for participating in the study.

If this is a third party study then as volunteer @myguy says, contact the leader of that study.

You would need to contact whomever is leading the study