You are only allowed to ask one question at a time so I edited down accordingly. Since the Earth (and as a result its atmosphere) occupies so much volume, it depends on whether you consider it as a whole or in small parts as to whether it is a homogeneous or heterogeneous mixture. If you consider the atmosphere of the planet as a whole, all of the major gasses (nitrogen, oxygen, and the other trace gasses) are essentially totally mixed up evenly to make a homogeneous mixture. There are no "pockets" of oxygen or any particular gas located anywhere. But on a smaller scale there are concentrated locations of water vapor in the form of clouds. These clouds are clearly visible as a separate phase from the rest of the atmosphere, thus leading to a heterogeneous mixture. So both descriptions could be applicable.