How much of an average modern car or truck can be fixed (as opposed to replaced) by an average modern mechanic? Does commercial vs. consumer make a difference? How about semi tractors?
It depends upon what you mean by “fixed.” Years ago, people would buy rebuild kits for their water pumps and do the job themselves, now they just swap them out for a rebuilt unit. It also depends upon what you mean by “average.” Someone with decent skills and a code reader (which retail for around $200 or so) can do a lot more repairs than someone with same skills who doesn’t have one.
There are certain things that the average mechanic shouldn’t attempt to play with, such as airbags. Those are way too dangerous to fiddle around with if you don’t have the proper tools and training.
Many of the things that the shade tree mechanic needed to fiddle with on cars are no longer used, such as points and non-self adjusting brakes. Other things are simply way too cramped to be worth doing it yourself. When the alternator went out on the Pontiac that I owned, I paid a mechanic to replace it for me once I discovered the insane level of contortions I had to go through to get the old one off. I’ve got a few back problems, and the amount of pain I probably would have ended up in wasn’t going to be worth saving the $50 the install cost me.
I thought I was fairly clear. I am asking about repair, which involves breaking the part down into component parts and making it work again (and possibly replacing one of those lower-level parts), as opposed to replacing the whole thing, and I am asking about the average hired mechanic, as opposed to people with more tools than brains. 
As an example, is it still common for transmissions to be rebuilt? That’s the kind of repair I’m talking about.
The dealer shops encourage bringing your recent models home for service. One reason they sell extended warranties. 
There are still many tranny service shops advertising thier services!
Thre are also many tuneup and brake services.
The DIYer is up against the complexity and compactness of the new models that are hard to get at for an individual without special capabillities and/or equipment. :rolleyes:
It depends.
Can I rebuild your alternator? Sure I can. Will I? Probably not, since I can buy a rebuilt from the factory for about the same or less than I can do the job, and it carries a 1 year nationwide warrenty.
Transmission? Varies with make. Some brands offer factory rebuilts (see reason above) others have the technicians do the work locally.
Steering racks, engines, rear axles, radiators, smog pumps are all the same. Either locally rebuilt or supplied as a rebuilt piece from the factory.
Let’s see. I know that these can all be rebuilt:
-Engines
-Transmissions
-Brakes, including calipers, boosters and master and slave cylinders
-Fuel injectors
-Alternators
-Windshield wiper motors
-Shocks and struts
Those are just what I have done, or have had done. I think you can also rebuild:
-Fuel pumps
-Water pumps
-I’m sure someone with sufficient electronic expertise and manufacturer info could resolder an ECU
In many cases, though, it is cheaper and WAY easier to just replace the part. Many things (like rebuilding alternators, fuel injectors and shocks) require very expensive equipment that the home mechanic is not going to have. Last time I checked, you couldn’t bop down to AutoZone and pick up a flow bench. 
This is what I expected. That said, I’m surprised how much of a modern commodity vehicle (the kind of vehicle the average schlub buys off the lot or as a used car) can still be fixed.
Is it substantially different for commerical vehicles, the kinds of vehicles large(ish) companies buy in fleets? How about semi tractors?
Fleet vehicles are 99% identical to the cars that you and I buy. Some of them may not have certain options installed or they might have additional devices to monitor things like usage, MPH, etc., otherwise there’s no difference. (The exception being things like police cars, which have extra electronics, tighter steering, and beefier suspension.) Things like delivery vans (say UPS trucks) are simply GM (or Ford) pick up truck chassises with a different body grafted onto them. Maintence on them is the same as for a pick up, at least as far as the mechanicals go. Semi’s are a different breed entirely. Because of their large size, it’s easier to rebuild components on them than it is for cars. No idea if that’s done or not, however.
As has been said; you can repair ANY part of a car. Its just that after a certain point, the labor to do this becomes prohibitive. take engines: to rebuild an engine 9and restore to original specifications), you would need to :
-disassemble and rebove the block
- plane the head and replace valve seats
-replce /regring crankshaft
-replace connecting rods
-replace/regrind camshaft
-check all tolerances
-re-assemble, and run test
All of this would cost considerably more than replacing the engine with one from a wrecked car.
The situation with modern cars is this: if major systems go (engine, transmission, airbags, lectrical harness), you sometimes are better off scrapping the car-the repairs are so costly that it isn’t worth it.
The entire powertrain (engine, transmission, drive axles) of a semi tractor is extremely durable, and worth fixing. So much so that it is often in decent shape when the body and other parts of the tractor is worn out or at least outdated. For this reason you can buy glider kits to renovate them. There can also be taxation advantages to doing such.