The idea that people āturn intoā angels when they die is advanced by well meaning people that insist on telling this to children. āYour granny is an angel in heaven watching youā.

This is not the teaching of the Church. Angels, are pure, non-corporal spiritual beings of high intelligence and free will. They have no bodies, do no procreate. Their number is unknown, and none are āunder constructionā.

When we die, if we go to heaven, we become part of the communion of saints which refers to everyone in heaven, which we acknowledge at every Mass, and ask for their intercession.
Good reading on the theology of Angels can be found on the EWTN online library.
But what others have said pretty well covers it.