> lspci is a bit more revealing, especially because you get to see where the dual 5GbE setup and Wi-Fi controller are placed–each seems to get its own PCI bridge:

That's how PCIe works. A PCIe port - both upstream and downstream - is a "PCI bridge". The link is one bus. A switch chip's "interior" is another bus. The next links are each their own bus again. One per port. There's no switch here, bus 0 ( / 30 / 60)is "in" the CPU, each port is it's own bus.

The more interesting thing is the PCI domain, the first 4 digits:

  0000:60:00.0 PCI bridge: CIX Technology Group Co., Ltd. CIX P1 CD8180 PCI Express Root Port
  0000:61:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8126 5GbE Controller
  0001:30:00.0 PCI bridge: CIX Technology Group Co., Ltd. CIX P1 CD8180 PCI Express Root Port
  0001:31:00.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL8126 5GbE Controller
  0002:00:00.0 PCI bridge: CIX Technology Group Co., Ltd. CIX P1 CD8180 PCI Express Root Port
  0002:01:00.0 Network controller: Realtek Semiconductor Co., Ltd. RTL8852BE PCIe 802.11ax Wireless Network Controller

Doesn't have to, though; it might be due to some technicality, driver bug, misunderstanding, whatever else.