Troubles in designing SIW to microstrip transition

Hello!
I would like to know whether the vias in the SIW are as an empty cylinder with only metallic walls connecting the top and bottom layers or they are totally filled with metall?
if the vias are not filled with metal are they filled with the same substrate dielectric ?or not?
I hope you could help me to clarify those confusions as I am a newbie in the SIW field.
thank you!

Consider standard PCB technology, opens vias with 15 to 30 μ metal thickness.

By the way, according to skin effect, filling the vias with metal doesn't change much, filling with dielectric neither.