Commodity auto industry supply chain parts? 400v is probably way overkill, and dealing with it is more complicated and expensive?

"Regenerative braking" is a feature implemented by the drive motor and controller, it's got nothing to to with the brakes.

Again, Tesla brakes are very conventional hydraulic devices and they work (and fail) like brakes in any other car you're going to drive.

The most recent Teslas (Model Y Juniper) have gotten a bit less conventional as they now have blended braking. ie: the brake pedal controls both regenerative and friction brakes, with the brake pedal "feel" effectively implemented by software.