Is this still true for the RPi 4? I believe for that model the Ethernet controller is in the SoC and not attached to USB.
Only Broadcom knows for sure, and they're not exactly telling.
In the
datasheet and
peripherals datasheet, the only mention of Ethernet refers to ETH_PCIe interrupts, which include PCIE_0_INTx, PCIE_0_MSI, etc., but also USB0_XHCI_0, i.e. the interrupt for USB0 (XHCI referring to an USB 3.0 controller). Whenever the 'Pi folks describe it, they say "it is part of the SoC and does not use the old USB 2.0".
The datasheet indicates that the Ethernet controller is not accessible from the ARM side –– it is not listed as one that can be safely accessed from the ARM side ––, so either it is connected to the USB0 (USB 3.0 controller), or to the GPU, or the datasheet is wrong. Because the RGMII pins are exposed on the GPIO header, and the board uses Broadcom BCM54213PE Ethernet controller (which connects to RGMII pins), it seems like it is connected to the GPU.
If only there was a proper block diagram of the connections on RPi4... For example, like the
Odroid-C4 block diagram showing the Amlogic S905X3 SoC connections. (The Medium Access Control, or MAC, is integrated on the S905X3 SoC, and is connected to RTL8211F PHY chip providing the Ethernet port.)