So does this mean the .dae is an explicit representation of geometry?
Also, does it make sense to classify HalfEdgeMesh into implicit/explicit ? Or is it just a data structure to handle any of these representations?
I would say yes that .dae is explicit. And the HalfEdgeMesh is also explicit. I would even argue the listed examples are also data structures, just HalfEdgeMesh might be a bit more specific as to the implementation.