Formations - NATO/West
While it would be possible to get into extreme detail with formations, I don't think it serves any purpose. All NATO countries and most Western countries, use roughly similar OOBs and TOEs. Whether a German battalion has two more snipers and one less mortar than an American battalion, is not much of a difference. And I don't think it ruins the immersion of anyone if there are tiny imperfections. As always, if you have more accurate information, do share! I'll make a separate post for Russian/Chinese formations as these are actually quite different. I'll use company as the smallest unit - again, the new ground combat model supports down to individual troopers but I don't have the time or patience to list every possible squad in every modern military.
Note that I am leaving out all HQ functions at Company level. It's probably best to keep using Battalion as the smallest manoeuvre formation. For this example, I'll be using American vehicle names. For British and German units, just replace the vehicle names and you're fine. All "infantry: personal weapons" are in addition to vehicle/gun crews.
Tank Company
14 M1 Abrams
1 M7 Bradley FIST-V
2 supply vehicles
Mechanized Rifle Company
14 M2 Bradley IFV
1 M7 Bradley FIST-V
81 infantry: personal weapons
2 supply vehicles
Artillery Battery
2 M7 Bradley FIST-V
5 supply vehicles
8 infantry: light bombardment (there are 8 towed 105mm guns, I can't think of a better way to describe them)
for mechanized version, replace i:lb with either M270 MRLS or M109 Paladin, number remains same.
Recon Company
1 M7 Bradley FIST-V
1 infantry: light bombardment (this is a 60mm mortar towed by a jeep and its crew)
48 infantry: personal weapons
6 infantry: light personal weapons (snipers and scouts)
1 supply vehicle
Weapons Company
1 M7 Bradley FIST-V
2 M2 Bradley IFV
4 M3 Bradley Scout
64 infantry: personal weapons
1 supply vehicle
Engineer Company
10 M1 Assault Breacher (yes, these are only for Armoured Engineer companies but it's best to just use these for all as Construction Equipment cannot be carried by Light Vehicles)
48 infantry: personal weapons
1 supply vehicle
Infantry Company -I think these only exist for Reserve units nowadays.
1 M7 Bradley FIST-V
1 infantry: light bombardment
3 infantry: crew-served anti-personnel
3 infantry: light anti-vehicle
90 infantry: personal weapons
1 supply vehicle
This is how US Army organizes itself nowadays and all NATO armies are either doing the same or planning to do the same in near future.
Combined Arms Battalion
2 Mechanized Infantry Companies
2 Tank Companies
2 M4 Bradley Battlefield Command Post
8 supply vehicles
Field Artillery Battalion
3 Artillery Batteries
2 M4 Bradley Battlefield Command Post
15 supply vehicles
Brigade Support Battalion
1 Weapons Company
1 Recon Company
1 Engineer Company
6 supply vehicles
Brigade Combat Team -old Heavy/Light brigades are gone and it makes no sense to differentiate between armoured and Stryker BCTs in Aurora context. So these would be Panzerbrigades in Netherlands/Germany and so on.
3 Combined Arms Battalions
1 Field Artillery Battalion
1 Brigade Support Battalion
This level requires a bigger HQ than the standard shown in vehicles list. I can't really determine what size until game is out.
I won't include generic divisions here because each country handles them differently.