- https://www.beyondlogic.org/usbnutshell/usb1.shtml a good overview of the USB 2.0 spec chapter by chapter from the perspective of a software side developer.
If you develop drivers (Software) for USB peripherals then you may only need to read chapters, 4 - Architectural Overview 5 - USB Data Flow Model 9 - USB Device Frame Work, and 10 - USB Host Hardware and Software.
You probably also need to know xHCI. The host-side USB spec.
- USB 2.0 Specification
- 4 - Architectural Overview: The USB is a polled bus. The Host Controller initiates all data transfers using token packets. The source sends a data packet, the destination replies with an acknowledgement packet.