How RTL-SDR samples signals

Hello to all, Please, can someone help me to understand how RTL2832U samples signals? I know it has two ADCs, one for the I (in-phase) component and other ADC for the Q (quadrature) component. In http://superkuh.com/rtlsdr.html the author states the signal is sampled initially at 28.8 Msps and then re sampled to present whatever sample rate is desired. But http://www.realtek.com.tw/products/productsView.aspx?Langid=1&PFid=35&a… says that 28.8MHz is the frequency of the crystal, not the ADC. I am confused with that, can someone help me? If RTL2832U is able to sample at 28.8Msps, why don't use the maximum sample rate? Maybe USB limitations? Also, all ADCs used in RTL-SDR dongles are inside RTL2832U? Or we have external ADCs? Thanks very much in advance, Lucas Ingles