Open mesh baselayer allows airflow to the skin for evaporative cooling. I’ve worn those types of baselayers for years and they both hold the jersey off the skin so you don’t get that sticky feeling and allow airflow between the structure of the mesh for evaporative cooling off of the skin.
Something like this:
Castelli used to make a similar base layer with that 3d effect, but now just have the thinner ones that don’t hold the jersey off the skin.