nay, the system can not be compared right to AACW for instance...
you can build
brigades
divisions
corps
expeditionary brigades
independent artillery batteries
siege guns
pioneers (for siege)
labour regiments (guess it was faster movement and some defensive bonus due to building trenches faster)
hospitals
fortress units of different size and quality
heavy guns for fighting passing ships
****************
Corps already have HQ-units and some artillery attached and could even work as independent stack
3* Generals are seldom, in case of US you have only one.
there is a function of abilities which apply to all the troops in the regional theater (limited region of commanded troops)
regarding leaders, best look for:
http://forum.paradoxplaza.com/forum/showthread.php?556020-Ins-and-Outs-of-Leaders&
division and brigades should be used with signal units and additional artillery brigade
due to the large provinces i used brigade only for securing supply lines, colonial wars and gaining military control (by staying around in a province for a while)
used also divisions only for besieging cities or encircling the enemy and cutting his supply line
while the corps blocks the enemy from moving and a couple of rested corps with plenty of additional batteries and some generals will do the fighting
Army Grooups will be created automatically as soon some corps are combined in a stack.
BE AWARE it might be called an army group, but alway have an eye on the black number describing the true combat power.
otherwise your army group may be repulsed from a single, but rested corps...
and furthermore, you have to buy (F3) reenforcements not only in case of losses, but also to use technical progressive arms and ammo, increasing your combat power