Now I understand OoBs, infact, that's what I was looking at when this question came to me. I'll use an example of why I'm confused.
Here seems to be one of the main Army Groups of the Soviet Union, as you can see there are four Armies on the side that you can click on to see the Corps and Divisions within them (as well as air units and uhh, I think Anti-Air?). But what were the Corps and Divisions attached straight to the Army Group doing? Was that so the general could have full control over them? To act as reserves? I assume the 'Fortified Region' ones are garrison?
If so (as in these units would have been directly under the main general, or if they were reserves), how would this work in battle, with some units attached to different branches (especially in largescale battles like Stalingrad or Kursk)? Would these units not go to battle unless the main general was commanding? Would they go to the battle but operate independently (how did that work with the artillery?), etc.
What was the reasoning for this? Asking here cause there is no /r/militaryscience or something, but there are a lot of military science people here.
As a side note, how accurate is this website and any recommendations for books on this kind of thing (as in for order of battles, for both operations as well as battles)? As I found some of these to be especially fascinating, particularly the Japanese and British ones. The Netherlands one, which was done by someone else was also really detailed.
Niehorster is one of the foremost western sources on OoBs and his information is generally reliable and accurate.
In this case, the army group commander has four armies and a set of corps, garrisons and other assets available to him.
An army was usually given a section of the front to defend, or attack along. Other armies were placed on either flank, or if no flank forces were available, light detachments or similar to keep the enemy from infiltrating behind your own forces.
It was common during this era to keep a reserve to use to beef up a defensive line in case of enemy attack, counter-attack an enemy breakthrough or reinforce a successful offensive. Since armies often ended up with sections of front that were unsuitable for large-scale armoured warfare (such as swamps, mountains or dense forests), many nations kept their armoured forces separate from army or corps command to allow the supreme commander to use them were they were the most suitable.
The Poles assigned small armoured formations to cavalry and infantry divisions to beef up their ability to fight enemy formationd and the French grouped their tanks either in armoured divisions or armoured brigades attached to army commands, to be used as suitable. the British used both army tank brigades to support the infantry, to be used by the army commander, and armoured divisions.
Here you see that the Soviets have enough tanks to both have two reserve mechanised corps as well as having several armoured divisions inside their armies in the Western Militiary District. This was often the case with the Soviets in the early part of the war against the Germans - their deep battle doctrine developed in the 1930s (rather similar to what the Germans used) dictated such formations, but by 1939, those who could or dared use such tactics had been weeded out in Stalin's purges of the Red Army. Those that did know and want to use such tactics mostly ducked and covered to not be singled out for the next purge. The Red Army froze in indecisiveness as a result.
A result of this was that the Red Army kept their 1. Armoured Division in the forests of Far Karelia facing the Finns for a long time before realising it would be of better use elsewhere.
So, to sum up - lower level units attached to higher command were usually.
Reserves (such as the Mechanised Corps in the link you provided).
Special troops for special operations (such as the paratroopers in the link you provided).
Local garrison troops that were immobile (such as the fortified districts in the link you provided).
Support troops with special equipment (such as the artillery and engineer formations in the link you provided).
Let us paint a scenario with the OoB you provided.
The commander of the Western Special Military District wants to attack German formations opposing him. He orders his air assets to step up its efforts to establish local air superiority and do air recoinnasance of enemy formations and positions.
The 10. Army is the best equipped, but it is not in the right position - he wants it to switch places with the 13. Army. He uses his 47. Corps and 17. Mechanised Corps to replace the 13. Army as it pulls out of the line. Then the 13. Army replaces the 10. Army in the line. The 10. Army returns to the depots to take on reinforcements (replacements), extra provisions and ammunitions. Its soldiers gets extra training in the type of terrain and enemt defenses they expect to meet. Its officers are briefed on the opposing troops, possible strong points, the strengths and weaknesses of the opposing forces and so on.
The commander attaches engineer formations to build pontoon brigdes, new roads for supply columns and other infrastructure to support the offensive of the 10. Army. Other engineer formations start clearing paths through minefields, destroying barbed wire hinders and other obstacles.
Much of the artillery assets are moved to the front with the 10. Army and attached to it. Its forward observers are deployed and they work with the air recoinnasance to pre-calculate a pre-assault barrage, aiming to destroy the enemy defences that the engineers cannot take out and destroy infastructure so the enemy cannot shift reinforcements or resupply their forces.
Finally, the 20. Mechanised Corps is attached to the 10. Army for the offensive.
The whole thing starts by the 3. and 4. Armies making their own, small, divisionary offensives to lure German reserves to their sectors instead of the real offensive. Then the barrage starts, lasting a for a few hours, all artillery pour as many shells as they possibly can over the enemy. Then the 10. Army moves ahead, breaking through and rushing into the enemy rear. The 17. Mechanised Corps is kept in reserve and might be commited to reinforce the most successful attack to keep it going. At the same time, one or two of the paratrooper divisions are dropped in the enemy rear to secure brigdes, roads and important infastructure (such as rail and road junction) and cause confusion in the enemy rear, and if possible, delay enemy reinforcements.
Right behind the troops are the engineers again, to repair roads and brigdes destroyed by the artillery and the enemy as he retreats, to allow reinforcements and supplies to travel through the conquered territory to keep the offensive going. The 47. Corps will be moved to protect the flank of the attack against any possible counter-attack and hold the occupied territory as well as built up defences for the 10. Army to fall back on, should its movement be reversed.