Oems for android apps often have their own contacts, dialer, calendar, and home apps. Usually this is to provide a differentiated experience, not to surveil you. Some are very good... Even better than the stock android experience. Others are terrible. Privacy is all over the board (except that most are using your Google account for storage so Google is almost always there). The vendor app stores often have device optimized apps and sometimes have better pricing. An example would be adding support for the curved edge on many Samsung devices...