Under most cases, and for most of the brake pedal travel, it is all Brake-by-wire. There is a mechanical linkage to the hydraulic brakes as a back up, but this will take considerable effort from the driver, and is not used normally.

Place a book on the passenger seat and brake.
If the book stays on the seat, friction pads ARE NOT being used.

If you brake hard enough for the book to slide off, some friction pad effort is probably being used.

-John