The MAC address is the Ethernet address of a network card endpoint, whether fixed or not. Multiple network cards, multiple MAC addresses. A single network card can also respond to more than one MAC address, or use randomized ones like in the case of your phone. They still tend to come with a factory fixed one, that is just used as a default when nothing else is changing it.

it can use the MAC address of the router itself (can't really see why you'd ever want to do that)

That's... are you sure is what it says? There are MDM managed networks where a router can push an MDM profile to a device, and set its MAC that way, maybe it's something like that?

A graphics card "shouldn't" have a MAC address... unless it has an output which can push Ethernet traffic (FireWire, HDMI HEC, etc.). A bit weird to have a licence locked to the GPU's whatever-port MAC, but possible.