I agree with you that bodies die.
But the soul does not. Depending on one’s life, the soul goes to heaven, hell or purgatory. It is very much alive.
The souls in heaven and in purgatory (In purgatory, they know they will eventually get to heaven after the harm done on earth has been atoned for.) can and DO pray for us if we ask them to.
Obviously the souls in hell cannot pray for us.
Remember the story of the rich man and the beggar, Lazaruse? The rich man asked to go back and warn his brothers about what was in store for them since he saw Lazarus the beggar in heaven. His soul was totally alive.
praying TO dead people is ancestor worship, not Christianity
praying to Jesus for the deceased is in keeping with Christianity though