My advice, don't try to learn on uphill battles.
Poland is probably hard / very hard unless you ally USSR (my only experience).
USSR vs Germany is hard. As in I have 500 hours in the game and I am cursing and making mistakes and the Germans broke my lines even though they are too stupid to even build enough tanks, they still broke my lines in the south.
This isn't EU. The numbers really matter. Poland has a weak economy. USSR is weaker than Germany in the short term.
Play Germany. That teaches you a lot. Because it goes from very easy to very hard.
The way to win USSR vs Germany? time, winter, attrition, good supply, keep making divisions, infantry equipment 1, 2 and 3 (make them all IMO), AA. keep deploying divisions. When it is bad, like 12 to 20width , shovels, engineers. Deploy them. Even if they are half trained. Even if they are missing a lot of crap. State AA in big states.
Pull all your tanks and motorized away. Save them for when the tide turns.
Don't bother with airforce. Controversial opinion but you won't be able to out build or out fight them in the air until 1944 probably. Whatever you have in the air, you have to use selectively. Like put your entire airforce in Ukraine for instance.
Two field marshalls. One with defensive stats. Use field marshall lines.
Fallback lines for extra troops. North, middle, and south.