Program A USB Port?

I'm confused , in the old days if I wanted to access a parallel or serial port I would simply set up the appropriate registers and read/write to I/O Registers. but USB is a whole different animal. Lets assume I plug device X into a usb port How do I find where the pc, or manufacturer has placed it? If it's a ddl then what is my fetch? if it's a virtual port where do I find the virtual irq and read/write port? What if it's a dongle, where do I find it's read registers? what if it's an external piano, where do I find it's sound out read regs etc.