Is skin or fabric more aero?
The simple answer for professional riders is that clothing is regulated by the UCI. Part 1 Section 3 of the Regulations of the UCI say,

When competing, all riders shall wear a jersey with sleeves and a
pair of shorts, possibly in the form of a one-piece skinsuit. By
shorts it is understood that these are shorts that come above the
knee. Sleeveless jerseys shall be forbidden. However, for downhill,
four-cross and Enduro mountain bike events,  BMX, trials and indoor
cycling, specific provisions are laid down in the part of the
regulations concerning the discipline in question.

The question of aerodynamics is more complex and is similar to the findings of swimming suits in Olympic competition: a properly-designed swimsuit is faster than swimming naked. For cycling, loosely-fitted or wrinkled clothing is much less aerodynamic than bare skin but well-fitted clothing can be more aerodynamic than skin if designed and fit well. This is particularly true for the upper arms and lower legs since they tend to present close to a cylindrical shape to airflow, but can also apply to the back and, to come extent, to the sides of the trunk.