That's a common trick in large generators. The field winding rotates, and the armature winding produces the output. So the heavy power doesn't flow through brushes.

So how is the field winding energized? There's a second smaller generator on the same shaft, with the armature winding rotating and the field winding stationary. This generates AC for the field. You want DC for the main field winding, so there are also diodes in the rotating system to convert the AC to DC.

The shaft floats on a self-pumping oil film. So there's no moving metal to metal contact in the machine. That's a good thing in something that's expected to run 24/7 for decades.

So now this power-station sized technology has been downsized to car size. Unclear if it's worth it, since the lifespan of cars is only a few thousand operating hours, not the hundreds of thousands of hours of a power station generator.