A PC host typically has a root hub, which is enumerated by the bus on which it is located during host startup. In our PC, the USB root controller is usually a PCI/E device, so it is enumerated and discovered by the PCI/E bus, and the address space of the USB root controller is permanently mapped in memory (visible through Device Manager).
A USB root controller typically includes both root hub functionality and root controller functionality.
The PC host communicates with the USB root hub, which then reports other USB devices or hubs connected to its hub. Other hubs report hubs and USB devices connected to them, continuing until all USB devices are discovered.
A root hub in Windows is a separate USB bus. If there are multiple root hubs in the system, there will be multiple USB buses.
Our PCs typically have 2-3 root hubs, usually including 1 USB 3.0 root hub and multiple USB 2.0 root hubs.
Ruigu Electronic 
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