To some extent, motherboard design is modular in that each design has the basic features provided by the processor and chipset plus the necessary support circuitry. Then, manufacturers add functionality with additional chips and support circuitry. If you compare different models from a series of boards all based on the same chipset from a single mobo manufacturer, you will see the degree of modularity in design.
Often, mobo manufacturers supply "OEM" boards for use in commercially mass-marketed builds from brand name computer marketers, and, often, these are based on the same modular designs you see in retail packaging; however, usually, some/many of the retail extras are missing, usually as a cost-cutting measure. For example, an OEM version of a board is likely to be missing the extra drive controller chip (thus only 6 hard drives rather than 10 -- 6 SATA + 2 SATA + 2 P-ATA/IDE), firewire controller, extra USB headers, etc.