Perhaps you do not know that according to Catholic Tradition, handed down from mouth to mouth, person to person, the Blessed Virgin Mary was visited by St. Michael the Archangel, who escorts people to heaven, that she would be dying soon.
All the apostles were miraculously transported from their various posts around the world at that time to her bedside. They were there to witness her death. So does it make more sense to you that Catholics believe in this dogma of the Assemption?
I have never heard of that but the Assumption is a traditional feast day of the Church.
Relics of saints' bodies were highly prized in the Early Church. Until Vatican II, every Catholic Church contained a certified saint's relic, usually small bone fragment, cemented into the marble altar. This practice originated in the early Roman church where mass was held in the catacombs and the priest used the tomb of a martyr as the altar for confecting the Eucharist.
If Mary had left a body at death, her relics would have been preserved as some of the most highly valued. There are no records of Mary's burial place or of her relics having been preserved or shared in the normal fashion.