My understanding of that from non-historian view:Well, that was a long time ago. Some of the lessons may have been forgotten. SOme of the Russian Army commanders may have been arrogant and underestimated their enemy, weapons and tactics. In WW2 the Germans were often under orders not to give up territory so it was somewhat easier to overwhelm them iwht numbers. It's not clear to me that the Russians today had the same overwhelming numbers on their side. I don't have any insight to the tactics of the Ukraine Army, but they may have been allowed to defend flexibly. Hit a Russian column in the front, force them to deploy to attack, then fall back and snipe from the sides, and do it all over again a few kilometers down the road.
During WWII, the first part (‘41-‘42) the Red Army was numerically superior, better armed , and was fighting a defensive war on its own territory, and still had their asses handed to them because their command structure and tactics were inferior. They improved by “survivor effect” — good commanders survived, learned, and adopted more decentralized tactics with combined arms. And Stalin let them make their own decisions. Then they rolled the Germans back.
Current Russian army is like pre-war Red Army — bored untrained conscripts, unprofessional officers. They have cool toys, but don’t know how to use them effectively. Power and decisions concentrated at the top again, for political power.
The Ukrainians recently underwent military modernization, using the latest Western tactics of combined arms and independent units.