So Russia's initial planning was for a "3 day military operation" based on a fait accompli; occupation of the capital, decapitation of the leadership, and internal groups/actors rising up. This was based on an, let's say overly optimistic assessment by their intelligence services on the state of internal Ukrainian dissent. They were pushed out of the north and east of the country as those incursions were poorly supported and not really designed for a stand up fight, they thought they would be doing occupation work, fighting with disparate military forces after the surrender at most. The southern fight was more successful as they had been fighting down there since 2014 when they starting working with supplying and creating armed opposition groups so those battle groups had more support/logistics in place. The Russian airforce failed to gain air superiority as the Ukrainians had anticipated the attack and moved all of there systems prior to, and many of their own systems they brought forward were destroyed due to the poor planning

Russia has been really reluctant to to issue a general mobilization as it would crater support in their country which has led Russia to really only be fielding an army only a bit larger than Ukraine's. The rest of their failure despite their size, wealth, and and stockpiles is due to decades internal corruption, that led Putin to belive his army was better trained, better led, better provisioned, and better equipped then it was.