Post-Combine Job Security, aka what sort of industry survived (or even thrived) by the time of HL2

After the Black Mesa Incident, the global economy must have fundamentally changed. The shift in human population towards the fortified cities during the Portal Storms must have crippled the rural industries, especially industrial-scale agriculture. Then the Seven Hour War happened, which would have done the same thing to those urban centers and vital industries. By the time Gordon Freeman was dropped into City 17, 20+ years have passed and we would have seen the economy stabilized under Combine rule. I'd like to infer from the in-game world which industry survives, and even thrived, under Combine rule.

Before we start, I'd like to think that at least around City 17 there is no such thing as a scorched earth policy being carried out as we see that most of the vital infrastructure remains intact. Maybe other places prior to the 7hr War was totally scorched, and we just don't get to see it.

Food

First and foremost, food production would have been subsistence based, done by individual citizens (discreetly if they have to), and any surplus would have been traded in the black market. We know this because we see greens being prepared in the BME kitchen, and by the fact that Lamarr eats grade-A watermelons.

As for meat, maybe headcrabs have become the new chicken. It could even be the case that standard rations citizens receive are just headcrab puree. Large scale animal agriculture ceased, and game animals would have been virtually poached to extinction.

Resource extraction

We see many abandoned mines, but maybe we've been only to one type of mine (coal mine), as a lot of resource extraction is still talking place because of the products that we can see (see Manufacturing section). This includes iron ore to make steel for the rails, and oil to fill that many explosive barrels that Gordon Frohman ordered. Maybe the reason we're not seeing them much is because the Combine is mining most stuffs in other places in the Solar System which they would definitely have superior technology for.

Trade

Global trade has all but collapsed except for Combine-essential commodities as trade infrastructure was destroyed or made unusable. Destroyed and unsafe highways and railways, and ports are all dry because of sea level drop. The remaining large-scale trade would be Combine-controlled, between Combine cities and for Combine-essential commodities, facilitated by the rail infrastructure that they opted to use for general transportation.

Local trade in the from of black markets thrive. I'd imagine that apart from food, the other stuffs traded would have been antiques from pre-Combine era or things smuggled out of Combine-controlled industries.

Human apparel, weaponry, supplies

Very obviously the Combine has been manufacturing the standard denim that its citizens wear, though as for what material it's made from it is probably synthetic. The weapons and armour and health supplies that the human combine forces use would be manufactured on-world, as it's primitive teleportation tech is reserved to transport more critical cargo.

Healthcare

IDK if the Combine cares about basic human healthcare more than it cares about turning them all into transhuman forces which has been demonstrated in HL2 that they have had the infrastucture for. The hospitals in HL2 doesn't look totally trashed so the Combine must has been offering basic healthcare through Breen's own volition, in order to maintain some level of support from the citizens.

Education, entertainment and arts

Officially nonexistent apart from the extremely small production staff that supports the Breencasts.

Infrastructure

The Combine has its own construction technology but it also seems to heavily use the rail infrastructure, so the rail industry chain must have been more or less intact. This includes everything in the chain, from making the steel for the rails, the sleepers, the rock quarries that supplies the ballasting, the specialist track laying and maintenance machinery and personnel, the signalling.

Also the bridges and tunnels seems to have been maintained by human tech instead of Combine tech so the civil construction industry chain is also intact and completely servient to the Combine. They do need to do this as the razor train is physically taller than conventional trains for some reason, and you can see developer oversight in some cases where the tall razor train clips through tunnel ceilings in C17.

Telecommunication technology and infrastructure might have been maintained and co-opted by the Combine, for Breencasts and local communication between Combine cities.

Manufacturing

As stated previously, some level of manufacturing is maintained, it's just the matter of whether it's Combine-controlled or Combine-linked. Most probably clothing, weapony, supplies, and ration production is Combined controlled, manufactured inside the citadels or other combine-built facilities. Manufacturing of Combine materials and tech (eg their blue steel, force fields, gunships, etc.) would have been combine-controlled.

Meanwhile the manufacturing of materials for the human-made-but-used-by-the-Combine infrastructure might have been outsourced to local companies, though unlikely.

Some interesting things that are still being manufactured (i.e. normal things you can see in HL2 that is obviously not 20 years old) includes oil and oil drums, wooden crates, soft drinks, office furniture, and some electrical and electronic products (lights, beverage machine and TV).

Law Enforcement

Last but not least, Enforcement: Civil Protection is a career path guaranteed to get you on the Combine's good side. You could go up the ranks (losing your humanity along the way) and would be surely, maybe, eventually, integrated into the Universal Union citizenry. Also, better rations (though how they make it much better than headcrab puree is unknown.)

Conclusion

In conclusion, there are still a lot of industries needed to maintain a civilization, and a lot of jobs around in the Half-Life universe. It's just a pity that the only thing we see in HL2 are enforcement officers on-duty, and normal citizens off-duty. It would be interesting to see City 17 during rush hour and at night where we might see a lot more people coming off of work. Imagine working as Combine Oil and Gas Engineer, or Breen's make-up artist, or Combine nurse, or Combine electrician, or being the guy that restocks Breen's water reserve vending machines.

Let me know if I missed something, or anything you have in mind.