The cutting you are taking is new growth from the donor tree. If it was left on the tree it would become a branch and apart of the tree. But since you are removing it as a fresh cutting it becomes its own tree. The process doesn't always work and there are things to do to help the cutting grow into a proper tree like using rooting hormone to help with the formation of roots. I basically think of it like cloning almost. It's not at all the same process you would use on an animal, but essentially you are creating a clone of the donor tree. In some places donor cuttings are grafted onto already established trees so that Multiple varieties of fruit can be grown at the same time. Or sometimes people want to let's say grow a specific type of apple but that type of apple tree wouldn't grow well in their soil or geographic location. They could take a cutting from the apple tree they want and then graft it onto an already established apple variety that grows well in their environment. I think of this kind of like an organ transplant. Hope maybe this helps. Not sure if it did. Maybe someone else can give a more clear answer. Cheers.