They don't rely on portal storms pretty sure, they can create portals themselves but only for extra-universal teleportation and they don't use Xen for it, the boarder world subway system thing is something humans figured out.
I think the Combine are just so overly evolved that they cant scale back enough to make simple technology, Judish literally says they just failed to factor in some Dark Energy equations which is preventing them from developing local teleportation by themselves
I was already saying that the Combine cannot do local teleportation (only universal teleportation) and that Mossman introduced was the one that introduced them to that technology. There are actually sources within the recent games that the Combine showed up after a week of the portal storms emerging to Earth’s Atmosphere. With portal storms being universal gateways (but in a much chaotic nature). It's just a matter of the Combine recreating or "tweaking" portal storms to either favorable conditions or destinations.
The whole plot of Episode 2 is to close the portal storm that’s hovering the destroyed citadel before the Combine could ever harness it; to re-established links to the Overworld and wipe out the entire Resistance Forces (under 7 minutes instead of 7 hours according to Eli).
The reason why I mentioned Xen it’s because it was the only place that has teleportation coordinates to Earth before the Nihilanth had been murked. It was implied through Gman’s dialogue that he and his employees lost or given up control Xen long after Gordon was put into stasis. "The borderworld, Xen, is in our full control…
for the time being!". Considering the fact that the Vortigaunts and the Nihilanth were on the run from the CMB, the empire would of likely stumbled into Xen without resistance from the Nihilanth.
