Voltages yes.. usually its all binary digital signals, running serial/parallel and following some communication protocol. Maybe you should have a look at something really simple/old like UART communication to get some idea how this works and then study next how this is scaled up over PCIE to understand the chat between CPU/GPU?
Or maybe not, one does not need all the details, so often just scaled concepts :)
Edit: Wait it is really already QAM over PCIE? Yeah then UART is a gross simplification, but maybe still a good one to start with depending on knowledge level?
Or maybe not, one does not need all the details, so often just scaled concepts :)
https://en.m.wikipedia.org/wiki/Universal_asynchronous_recei...
Edit: Wait it is really already QAM over PCIE? Yeah then UART is a gross simplification, but maybe still a good one to start with depending on knowledge level?