Oh..... the metric / SAE debate *rolls eyes*
Here in canada we still use half metric and half SAE... road signs in metric but still use Pounds to measure most weights. Go Figure.
As for what parts of cars are in metric or SAE I would love to present you with a 1984 chevette diesel... Chevettes were actually a german design (opel kadett rev.C) and much of the car was metric, some of the parts unique to the US market were in SAE, and anything relating to the engine was Metric (but used different common sizes as to the german metric designed parts as it was ISUZU!!)
Driveshaft companion flange was held onto the torque shaft with a 24mm bolt, and the u-joint attached to it with 7/16" bolts! and the bearing on the shaft was also held to the body with 15mm bolts. Many bolts were 13mm OR .5" which was rather frustrating on many occasion.
Page one of the factory service manual has a warning telling you that multiple socket sets will be needed.
BTW... chevettes were designed in 1974 origonally, and not released into the US market untill 76
This kind of "what system do we build cars with?" is just moot, all depends on the engineers and country of origin. Today, most cars are all built with metric with VERY few exceptions. Regardless of country of origin.