How does a computer know about the many devices not known by the System ROM?

The OS can load device drivers on startup to probe (examine) memory locations for answers from hardware that the System ROM can't know about (never programmed in or too new for it to understand it).

Device drivers are how computers can know about devices that are not in the system ROM. The drivers would scan the addresses where the devices are found and load if the devices are present. Drivers are a way to artificially extend the functionality of the ROM BiOS (or EFI). The OS loads the system drivers.

A metaphorical example might be a literal city bus driver. The driver starts the bus, it runs, and he adds people, but the bus itself doesn't know anything about people, just about running. The driver makes the difference. In this analogy, the Bus is the computer with System ROM, the driver is the operating system and/or device drivers, and the people are the devices being connected or disconnected.