In the Linux kernel, the module loader is woken up when a new device is
detected. It's passed a "modalias" string, which identifies the device and
looks something like this for USB devices:

usb:v058Fp6387d0103dc00dsc00dp00ic08isc06ip50in00

You can find your device modalias (and the HCD driver) using the following command:

udevadm info --export-db

This is the section related to my USB drive:

P: /devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0

E: DEVPATH=/devices/pci0000:00/0000:00:14.0/usb3/3-2/3-2:1.0

E: DEVTYPE=usb_interface

E: DRIVER=usb-storage

E: ID_MODEL_FROM_DATABASE=Flash Drive

E: ID_VENDOR_FROM_DATABASE=Alcor Micro Corp.

E: INTERFACE=8/6/80

E: MODALIAS=usb:v058Fp6387d0103dc00dsc00dp00ic08isc06ip50in00

E: PRODUCT=58f/6387/103

E: SUBSYSTEM=usb

E: TYPE=0/0/0

E: USEC_INITIALIZED=530904794

This string contains the device class (usb) and class-specific information
(vendor/product/serial number, device class, etc). Each kernel driver
contains a line such as:

MODULE_ALIAS("usb:...")

Which must match the usbalias (wildcards are used to match multiple
devices). If the modalias matches one that the driver supports, this driver
is loaded (or notified of the new device, if it's there already).

You can see the supported devices (by modalias) and their associated modules with

more /lib/modules/`uname -r`/modules.alias

If you grep for the usb-storage device driver, you'll see it has some
specific devices it supports by vendor and device ID, and will also attempt
to support any device with the right (storage) class, no matter the
vendor/device.

The USB modalias can be decoded like this:

usb:

v   vendor_id (4 hex)

p   product_id (4 hex)

d   revision (4 hex)

dc  class (2 hex)

dsc subclass (2 hex)

dp  protocol (2 hex)

ic  interface_class (2 hex)

isc interface_subclass (2 hex)

ip  interface_protocol (2 hex)

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