Is Thunderbolt 3 like the rest of the USB-C protocols in that dedicated pins are used for USB2? If so, the bandwidth doesn't need to add up -- the cable carries the Thunderbolt protocol and USB2 separately.
Based on the D+/- pairs I believe you’re correct. On Titan Ridge and later add in cards USB 2 is expected to be passed through via a motherboard header as the controllers lack a dedicated USB 2 chip.
With Thunderbolt the USB controller is built in to the display, with full bandwidth available to each USB port.
But if the dedicated USB2 pins were being used, it would just be acting as a USB hub, and the total bandwidth on all ports would be limited to 480 Mbps?
No. TB3 still stuck with only using PCIe-USB3 controllers over tunneled PCIe.
The other replies are right on the host side, where newer TB3 controllers pass-through USB2 from the chipset. But that is only used for non-TB connection modes.
Same with TB3 Titan Ridge Docks. They are wired to the USB2 pins, because in legacy mode (DP Alt mode + USB3) the USB2 will be physically on those pins together with the 2 high speed wire pairs used for USB3.
Only USB4 started using those USB2 wires on USB4 connections.
