Very interesting discussion.
It's understandable that - with the supply flow being abstracted - some details aren't on the cards.
In some armies, Bns had to draw from (the very small) divisional depots, for instance. In the German army, divisional depots never had a fixed place, and not a fully-fledged supply vehicle pool, either. The Division could only store the amount of supplies that was needed to equip the frontline units with the first loadout (before going into combat). The Corp was not involved in the (re)supply process at all.
The Army (unit) set up and maintained fuel depots in each division's AO, which means that it pushed fuel supplies to the divisional area of operations.
For food/water supplies - in non-motorized units, the Bn HQs combined the Coys' supply columns no. 1 (one horse-drawn vehicle) of each company and sent them to retreive and distribute food/water supplies. The Coys' supply columns no. 2 (one 3-ton truck and 1 Krad per Coy) were usually employed/combined by the Regiment and used to retreive and distribute ammo to the Bns.
So food was handled slowly and ammo was shipped double time, if trucks were available/combined, basically
An artillery regiment didn't have supply columns, as each of its Bns had their own columns: a light artillery Bn had 1 light supply column (motorized, in some units partially horse-drawn) with a total capacity of 36 tons, heavy arty Bns had 1 light supply column (motorized, 28 tons total capacity) in each battery, IIRC.
Ammunition supply chain: the industry delivered the ammo to the Wehrmacht's ammunition institutes. Ammunition was often delivered separated (safety measure), so the institutes then had to assemble the parts. The institutes also served as QA entities, so they also verified functionality/quality and adherence to production standards. The ammunition was then transported (train cars) to the ammo depots of the Army (unit). These deliveries were directly hauled to district depots near the divisions in exceptional cases, only.
Usually, the supply columns of the Army or the Division would then draw ammo supplies from the Army ammo depot and haul them to the divisional issuing point.
The light supply columns of the regiments/of independent Bns then drew the ammo for their subordinated units, and delivered the ammo supplies either to the fire bases of the artillery batteries or to the frontline units' ammo supply points. Combat vehicles could pick up ammo supplies, ammunition carriers (as in men) picked up ammunition for the infantry (at the frontlines), at these points.
If the frontline conditions allowed for direct supply runs, supply vehicles were sent right to the frontline.
The US Army used well defined procedures as well, which can be checked in the particular field manuals. Jim pointed to a number of quite interesting FM-manuals, in the past.
The Germans used a wide range of vehicles to deliver ammo to frontline (trench) units, they converted/used a number of tankettes and ammunition carriers (mostly of French/Belgian, British and German make) to save precious trucks and to be able to cover the last say 200 to 600 meters under combat conditions. They even modded such vehicles, so that ammunition could be dumped into trenches and strongpoints under moderate/heavy small arms fire by using a handle from inside a carrier (pretty much like in a lorry), where then the cargo bed could be raised and where the ammo then just slid down into the trench, for instance. This regime reduced manual work and troop losses.
British units used bren carriers, converted Vickers tankettes or similar vehicles for such jobs, afaik.
That said, it's quite clear that it'd be quite a job to render such details.
On the other hand, and with the existing system, you have a situation where trucks are sent into combat, technically.
It is true that say the Russians and the Germans lost huge amounts (relative to their total amount of trucks produced/captured/received) of transport/supply vehicles, but it's also a fact that a large amount of these supply columns were destroyed by situational fire, eg. unexpected enemy advances, air raids, artillery interdiction fire (accurate Allied fire was rather rare, at least until autumn 1944) etc., or by planned interdiction/S&D missions - performed by fast motorized units or by partisan units forming raid parties, and not by single enemy units occupying single supply routes.
Generally, the several armies, especially the Germans, tried to avoid sending precious cargo space into combat zones.
The German Army lost quite an amount of supply trucks through attrition (wear & tear, breakdowns) in 1941- when they had to cover huge gaps (large distances that had to be covered with truck transports because the Russian railway lines had to be converted, first, the transport pool used for this extra job - in the main - comprised of a huge amount of Allied trucks captured in 1940), but also some amount during their onslaught in France in 1940, already, with another huge loss of cargo space during the Russian counterattack near Moscow in December 1941 and in the Stalingrad pocket, means through enemy action, of course. The Germans never managed to replace the loss of those captured vehicles and the subsequent loss of trucks when the Russians started their counterattack near Moscow, their production output was too low, production and assembly of truck parts were too slow.
So, except for unexpected enemy actvities (counterattacks, raids, etc.), supply units tried to avoid the frontlines (especially the horse-drawn ones) wherever possible. Yet, in the game, everybody and his mom seem to be eager to bring their vehicles to the (combat) party. If you investigate ingame and try to find out who's sitting on your supply route, you can very well dash into remote woods and past elevations off-route, without seeing any enemy nearby. It feels like enemy units resting 1 or 2 squares away from your supply route are destroying a vital percentage of your supply trucks, even tho they didn't even move for some say 12 hrs. Now, I may be wrong here, but that's how the system felt at times.
Another detail: with the details about the German supply regime, it is quite obvious that putting the transport pool into a giant base attached to say a Division doesn't do the historical regime justice.
Basically, if say 2 Coys lost their supply truck elements, then the ammo supply level of the sister Coy was affected, too. The regimental officer in charge of the combined supply columns no. II (from the Coys) then either had to spread the ammo evenly (so that all attached Bns received less than the required ammo deliveries) by using the remaining truck, or the Coys' food supply elements (reminder: horse-drawn) would have had to fill the lost trucks' (2) roles and draw ammo from the distribution point, until replacements for the trucks could be acquired.
With the latter option, such Coys would have been undersupplied ammo-wise in combat situations due to the low speed of the horse-drawn vehicles, for the time being, because the pretty small Bn column could only support the Bn HQ.
Since the Coys' food supply columns were combined and controlled by the Bns, the Bn officer (or the division's quartermaster) in charge of the supplies could also opt for using parts (or the whole) of the food supply columns to haul ammo to the Coys, but then the food supply of all subordinated units would have been severely affected, of course.
In the game, the emergency supply runs seem to counter/rule out such historical effects/outcomes, partially at least, which leads to somewhat unrealistic results, imho.
In any case, the game's current supply system doesn't represent historical German or US supply regimes, and since supply units are not rendered the actual cargo pool losses are definetely kind of off-kilter, imho, which means they appear too high.
