Evaporation from oceans together with other water bodies in the air and transpiration from land crops and animals in to the air. Oxygen dihydride is yet another way of referring to water, but modern day use frequently restricts the expression "hydride" to ionic compounds (which water is not). A diagram https://kevalaniru.com/why-boxed-water-for-kids-is-the-healthiest-choice-for-family/