The Z4 has a "skid plate" by design. I don't believe the E36 is similarly constructed. It's called a 'reinforcement plate', it's made of thick-ish aluminum, it's held in place by quite a few quite stout bolts with quite high torque settings, and it not only protects the underside but also ties various bits of same together. Oh and the front jacking point sticks out the bottom of it.
https://www.realoem.com/bmw/enUS/sho...diagId=31_0779