and why is there still a little bit in oases
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Underground, which is why it can still be found in oasises. which are very low-lying areas where the water can reach the surface.
Give it enough time all water that falls on land will eventually either flow into the ocean, settled down into the ground, or evaporated away.
There are things called aquifers, underground stores of water that can’t sink any lower, because the rock below them won’t allow water through so you get an underground puddle, where the land dips low enough compared to the surrounding area the top of the “puddle” can break through to the surface again. https://youtu.be/xbUfVLxYVcE
When deserts used to be wet, they had lakes and rivers. Rivers carried most of the water to the ocean. Some of the water seaped into the ground over time and became aquifers — specifically, they became fossil groundwater because they are no longer being recharged with new rainwater, and so as we use them up (e.g. in places like Yemen, or to a lesser extent the only semi-renewable [Ogallala Aquifer in the US](https://en.wikipedia.org/wiki/Ogallala_Aquifer)) they simply go dry over time, unlike renewable aquifers in non-desert places.
But the most important answer to “where the water went” primarily is that the water just stopped coming as climates changed over many many years. sometimes this happened over eons due to continental drift, sometimes it happened somewhat more quickly (i.e. over a period of tens of thousands of years) due to long-term cycles in Earth’s average temperatures and the effects of global lower temperatures (or higher temperatures) on global weather patterns (things like ocean currents and ocean water temperatures having major effects on what areas receive monsoons and what areas never receive rain). So once the rain stopped coming as the climate changed, the water that was there was carried by rivers to the ocean, seaped into the ground, or evaporated and was carried away on the wind.
Water is constantly leaving via run-off, river flow, soaking into the ground, and evaporation.
In places that have lots of water, the amount of water falling from the sky as rain or snow is roughly equal to the amount of water flowing/soaking/evaporating. Sometimes there are short period of time where there is too much water falling too quickly and the water builds up. We call that flooding.
In deserts, the water stopped falling due to changes in climate and so all the water flowed/soaked/evaporated away.
However, the water that soaked into the ground may have been stopped from soaking down further by solid rocks. In places where the ground dips down below the level of this underground water that has soaked in, it’s available to us at the surface.
In deserts, we call them oases that water is generally not replenished and will eventually run out.
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Water doesn’t stick around on land. It is constantly evaporating or flowing away. In wet places, new precipitation falls to make up for the water that leaves. When places become deserts, not enough new precipitation falls and so the area just dries out.
I could be wrong, but iirc deserts like the great Basin desert in Utah/Idaho/Nevada in the western United States used to be a giant inland Sea formed by the melting of glacial ice sheets which used to cover huge swaths of Canada and the Northern U.S during the last Ice Age. Since these glaciers were not renewable reserves, then once they melted, that was it. The great basin filled with water and the weather patterns/rain shadow from the Sierra Nevada/Cascade mountain ranges did not provide enough rainfall to replenish the lake, so over thousands of years it evaporated, leaving only the Great Salt Lake and Utah Lake as the last surviving (and rapidly drying; thanks climate change and 20 yrs of drought) remnant.
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