How are the patterns of groundwater distribution expressed?

Groundwater is located in the earth’s crust at the first aquifer from the surface. Precipitation replenishes groundwater. After heavy rains or melting snow, their level increases. In hot summers and cold winters, groundwater reserves are not replenished and the water level decreases. Since in different parts of the planet the climate differs in different amounts of precipitation, the distribution of groundwater is uneven. The drier the climate, the deeper they are. For example, in the Sahara, wells are more like missile silos. But where does groundwater come from in the Sahara, because precipitation is very rare here? Scientists believe that they were formed in past geological eras, when the climate of this land area was more humid.