Game wise the same things happen every game. There is no pearl harbor so the US has a fair fight vs Japan, even can take them out within a year if they desire. Pearl Harbor should kinda be like the great purge is for the Soviets... devastating in the near term and mostly inevitable.
The German offensive against the USSR goes no where because the red army is massed at the border (that area should be demilitarized to reflect the surprise).
A lot of this depends on which version you are playing. In 1.8x I've never seen the USSR win against AI Germany without tangible support from the Allies (an active D-Day or a boatload of volunteer divisions). In prior versions, the USSR tended to win against Germany even when fighting alone. When they patched the AI to get it to garrison its VPs to prevent paratrooper meme shenannigans, Japan started to struggle against China because it would garrison all the cities and ports of the Home Islands against an enemy that didn't have an airforce, which didn't leave enough divisions to effectively push the Chinese back. I've seen the US bleed itself white trying to push the Japanese out of the Phillippines, and I've seen them naval invade Kyushu in January of 1942. When you play as a minor and focus on regional expansion and stay out of WWII you learn a lot about what the AI will do unmolested. The AI is balanced to play against the AI (for the most part), and every time the devs try to strengthen one aspect of the AI it unbalances something else. Germany needs to be able to beat the USSR so that the Allies player has a reason to get involved. Japan needs to be able to beat China, but not so quickly that they conquer the country by 1939. The invasion of Iraq and Iran is on the UK focus tree, but it is in the "Home Defense" branch of the tree and the AI almost always takes "Global Defense" instead (why invading two foreign countries is filed under 'home defense' I can't explain) presumably because the AI is told to think Global Defense is the better branch.
And so on...
TL;DR A lot of the un-historic stuff you see in the game is down to the AI being the AI, and its behavior will change from version to version.