Are thru-axles generally hollow or solid? Need to know if these thru axles (of any size) have an internal diameter or are completely solid. How common is each type?
Thru Axles are a standard for wheel fasteners that was introduced fairly recently for MTB and later for road bikes. Today thru axles come typically in 15 mm or 12 mm diametres.

If you mean thse, they are indeed always hollow. Any solid bolt with such a large diametre would be excessively heavy. Even when using light magnesium alloys.

The whole innovation of thru axles revolves around that when having a structural element of sufficient size one may do without its core. In other words, a tube of a large radius is under many loads stronger than a slimmer rod of the same mass and material. In the case of a hub axle one can rule out loads (e.g. shearing) where a rod would be stronger.