Trees grow layers on the outside, just underneath the bark. This is the only part of a tree trunk that is actively growing and is part of the plant’s circulatory system. All the older wood toward the center of the tree is basically only structural support.
There’s a technique called “collaring” used to remove invasive trees, if you cut a shallow, 4 inch tall ring from the bark around eye level, everything above that ring will die, because you’ve severed the circulatory system between the roots and everything above the collar