Uluru is made from a sedimentary rock, a coarse grained sandstone.By contrast, Kata Tjuta is made from a sedimentary rock called conglomerate – a mix of gravel, pebbles and boulders cemented together by sand and mud.Their formation occurred over millions of years; initially the material was on the surface then covered, then the upper layers were weathered and worn and with some faults fractured and folded the land, so that eventually what we see today is a small portion of the rocks. Just as the majority of icebergs are below water, the majority of the substance of Uluru and Kata Tjuta is out of sight, underground.
Superb diagrams here show the process and allow you to see that the seemingly large Uluru outcrop is, in fact, the tiny end of a sweeping many kilometre length of rock that continues below ground. A further fact sheet can be read here. Since appearing above ground both rocks have continued to be eroded by climate and weather. Nothing rushed of course. I suggest both will be around long after the human race is extinct.
Statistics for those who like them: the height of Uluru is 348 m above the plain (863 m above sea level); Kata Tjuta is 546 m above the plain (1066 m above sea level); fauna species = 21 mammals, 73 reptiles, 170 birds and 4 frogs; flora species= greater than 400; average rainfall = 307.7 mm per year; temperature extremes=up to 47 degrees in summer and down to -7 degrees in winter.