This topic can be divided into two:
In any sizeable SoC it is likely that you will not build everything yourself. Various blocks of ‘Intellectual Property’ (IP) are available, freely or at a price, for incorporation. into other designs. This is the business model of, for example, ARM Ltd; they do not make chips themselves but license processors and other designs to Apple, Nokia … These equipment designers will purchase rights to use IP blocks from multiple sources and integrate those with their own application-specific logic to make a purpose built chip.
IP blocks typically come as ‘black boxes’ and it is the function and the interface which are of interest to the developer. Having some standard interfaces allows blocks to be composed easily.