The KML Conversion documentation (KML conversion—ArcGIS Pro | Documentation)
mentions that "The layer description will be used as the pop-up display content for the folder."
How do I set the layer description? The closest thing I could find to that was the description in the metadata but the description I put there does not carry through to the KMZ
Update to this, ArcMap has a description section when you view a layers properties, but Pro does not for some reason. I was able to set descriptions for my layers in ArcMap, and then import the MXD into pro and the descriptions persist when exporting a KMZ from this. However there is no way to view of edit this description in Pro which is not ideal. ESRI, please provide access to the layer description in Pro that is available in ArcMap.