In Monsters Manual we can read that a lich cannot be destroyed if its philactery still exists, and that it is similar to magic jar:
In all cases, a lich will protect itself from annihilation with the creation of a phylactery in which it stores its life force. This is similar to a magic jar spell
In its creation we can assume the philactery is used to allows possession of the own mage's body, but if a lich has its body destroyed, which body its soul will possess? A stored dead body? Or anyone who approaches?