There are 2 ways for an individual unit to gain experience. The first is combat experience. I am not sure if this requires that the unit actually sustains step losses, or if simply being in a battle gives it a chance of gaining experience. But I think it only requires the unit to be in a battle. At least this is my experience with U-Boats as they seem to gain experience from sinking MS and escorts even if they don't sustain any losses themselves. I also believe that there is only a chance of gaining experience from each battle and that it will only increase it's experience by a maximum of 1% per battle.
The second way is for a low experience unit to sustain casualties and then have those casualties replaced by more experienced replacements. So you say that you still have Russian units with 30% experience, but that your Russian National Experience level has reached 50%. If that is the case then if you have a 30% unit lose half of it's strength in battle than that half will be replaced with 50% experience replacements; so once it is fully replaced it should have 40% experience. At least that is my understanding. Of course, the opposite is the case if you have a high experience unit take casualties. This is why those German air units that begin the game with 70% experience slowly degrade down to lower experience as they take losses.
This is how I understand it works also.
The paradox for this system as relates to the USSR in 1941, is that losing battles increasing XP more than winning them.
A unit gains direct combat experience very slowly, approx 1% a turn it seems to me (if they are fighting), whereas national default morale quickly rises to 50%.
This means that the best way to gain morale is not to WIN combats, but to LOSE steps.
For example, you have 2x 30 step mech unit at 20% XP.
The first one takes part in 3 successful attacks, losing no steps, and gains 3% XP= 23%
The second unit gets savaged in 3 unsuccessful defences, and loses 25 steps. However national morale is currently 50%, so the replacements it receives takes the unit's XP to 45% once it is rebuilt.
So the unit that performed worse, took the most losses, and received the most 'raw' recruits re-emerges as far more powerful force than the one which performed better and took no losses and needed no replacements.