Typically, graphics cards are connected via PCIe slots or Thunderbolt/USB4 interfaces that support PCI Express tunneling.

[delokaver] figured out how to use these 3G cards over USB instead ... The modem relies on the 3.3V power from the Mini PCI-e slot instead of the 5V from USB. No problem, just get a low-dropout ...

This showed that the Raspberry Pi would happily talk with a VL805-based USB 3.0 PCIe expansion card, as well as a Realtek RTL8111-based Ethernet card, but not a number of other PCIe cards.