The SiLabs USB to UART/Serial bridge is used on the WICED Sense evaluation kit.
An FTDI USB to UART/Serial bridge is used on the TAG3 development kit.
The role of each is to provide an interface to a PC for development/debug, but they can also be used in development to program your custom board using either the SDK or command line tools that are described here: Programming the TAG2/TAG3 Board using command line tools
Note that many do not include these devices on their production boards for size and cost reasons, and instead use an external USB to UART/Serial cable such as the one described in the blog noted above within Section 4: USB Enumeration Connection to the PC USB Enumeration of the BCM2073xTAG Board
In addition to the Bluetooth Stack, Profiles and all necessary drivers that are loaded into ROM, the BCM20737S module (and all other versions) ships with core firmware and a program used for factory testing already loaded (EEPROM) as well.
Note that in order for the custom module to be programmed externally, the HCI UART will need to come up in programming mode. The firmware uses the state of HCI Rx and the prescence of valid images on the NVRAM to determine the mode. This is described in numerous threads found throughout the site.
There is a grouping of these threads here: WICED Smart Bluetooth Forums
Under the 'Manufacturing and Test" category found on the left hand side of the page.