Throughout Half-Life 2 and its episodes, we never get a good look at the Combine outside of its Earth garrison, and even then the view of the portal above the Citadel only gives us a small glimpse of the overworld. What we do see can only be in the context of what the Combine does to worlds it conquers and drains of resources, yet from that we can gather a lot about how the ruling force acts, what its priorities are, and even what I think it actually is.
Architecture
The first thing one might notice about Combine structures is that they seem to be slapped on with little regard for the underlying building it now occupies. Same exact metals and general design philosophy of interlocking pieces that may or may have humans in mind, and every last bit of the inside repurposed for their needs. The Citadel is the ultimate example, with narrow walkways and no guardrails, factories taking up entire levels, and even where Breen seems to work is very utilitarian. Not a single bit of the Combine's architecture was ever designed for anyone else, only what it seeks out of occupying that area.
Vehicles
Being that Half-Life 2 takes place on Earth, we only have human vehicles to work off, yet what can be seen says a lot. The APCs don't look like they were designed for anyone to drive, as the front end is too thin and only the back compartment might be big enough to get inside. Hunter-Choppers are more of an imitation of human helicopters, misinterpreting how many rotors one actually needs. Razor Trains look just as brutalist and unfriendly as everything else the Combine makes, designed only to use human train rails and nothing else. The Combine builds odd mimicries of familiar vehicles that only resemble their inspirations at a glance, but a closer look reveals just how off the mark they are.
Earth occupation
Bleak doesn't begin to describe how the Combine runs Earth as we see it in HL2. Humans are forcefully settled into cramped apartments with little furniture, everyone wears the same things with no room for self-expression, police brutalize people for daring to speak up or simply because it gets them perks, and if you rise against the occupiers you're killed at best, and shipped off to become a Stalker at worst. Even if you do get a job, it's only in service to the regime as a worker repairing their stuff or beating somebody as Civil Protection. Everyone eats packets of only the bare essentials a human needs with none of the flavor or appearance that makes food appetizing. Whatever the higher-ups of the Combine hierarchy want from humanity, it definitely does not involve considering what a person needs for enjoyment in day to day life.
Military
The existence of enemies of HL2 like the Soldiers and Synths give depth into how the Combine invades a world, enforces its rule, and makes use out of the local life. Striders are said to be scanned from their homeworlds and built as war machines, although the one Dog fights having a brain suggests this might not be the case for every one. Regardless, the Synths show that the Combine can't create its own armies and instead copies from what it deems the best lifeforms for its purposes. Creativity and innovation is not exactly the empire's forte.
How Breen views them
This might seem odd but I promise it makes sense.
In his Breencasts, Breen frequently sings the praises of the Combine, calling them Our Benefactors and attributing the idea of them being benevolent alien invaders even as everything around him proves otherwise. He says the Citadel was "so thoughtfully provided" by them, but who knows if they were being actually thoughtful or just dropped it there because that's where they thought it would have the most use? The suppression field is painted as a way to kill human instinct, and make humanity progress to immortality. Why would a multiversal empire create a planetwide forcefield that shuts off the ability to reproduce if it wanted a species to live forever instead of just giving us a recipe for an immortality vaccine?
Clearly Breen doesn't know as much about how the Combine works as he thinks.
Teleportation
The biggest lynchpin in this theory is the fact that the Combine is stated to not know how same-universe teleportation works. Mossman says they can only tunnel into another universe and have to transport stuff the old-fashioned way from there. That's why they are so desperate to learn how to make same-universe portal tech, to the point where the one in Nova Prospekt is seemingly very experimental, with its second usage destroying the device. They can't develop teleportation on their own, and they have to steal it from another species so they can make conquering easier.
Conclusion
So far the Combine has been described as a very sterile force, not just in how they build but in how they act. They can only take from other dimensions with evidently nothing being an original creation of theirs. Humans under their rule are only given what the Combine thinks humans need to survive with no regard to making us happy or comfortable. Their puppet leader believes they have humanity's best interests in mind when he's probably just as disposable as everyone else. Synths are creatures that barely resemble their original selves and built in factories to take over planets.
The Combine is cold, unthinking and particularly unoriginal, only seeing fit to drain resources from worlds and leave with more numbers to its armies when it's done. Parasitic and cancer-like are both apt descriptions, but I think it goes deeper than that. Its creative sterility is nothing like what a cancer does, yet there's seemingly a plan to it all. A logical kind of plan.
And what thinks in terms of logic?
An algorithm.
Behind the Combine is an AI.
