Some observations on now reserves and army groups function in game.
You can reinforce your army with divisions from other armies or even create a separate army, like creation of a separate 18th army before Tallinn.
You can reinforce your army from reserves if there is anything in them. There is an easier way, if new forces have arrived before the battle, you just see them deployed on the battlefield.
Sending forces away from your fighting army is more tricky. If you send a force away from your army it ends up in OKH reserve.
Also, if you exchange units both ways the forces you receive will arrive but the ones you send away end up in OKH reserve and the other army group must take them from there.
In game terms, this does not matter, unless you have found a map with the OOB you need and want to deploy the divisions exactly as they were. And even if you absolutely want to send 12 th panzer division to Tikhvin and get 1st panzer division from AGN instead you can just click OKH reserve in reinforcements.
Also, I do not believe the transfers happened at the same time, that would make no sense.
In a longer campaign you just cannot keep track of all divisions, there are too many of them.