Google is getting their count from a Nature or Scientific American article which mentions that the number of trees is calculated from "421,529 measurements from

Google is getting their count from a Nature or Scientific American article which mentions that the number of trees is calculated from "421,529 measurements from 50 countries on six continents". Fires also clear away old growth so new trees can have a chance, and trees native to fire-prone areas have evolved fire resistance. In addition, some pine cone seeds are released by fire. As for the net loss or gain, you'd have to subtract how many trees died in the fires from how many new saplings there are annually. The world is a big place. It's perfectly possible for a billion trees lost in one region to be offset by a billion new trees elsewhere.