It's an interesting story. The D800 was built of magnesium alloy. It had a problem with cracks at the back of the mirror box, magnesium alloy being quite brittle. For the D810 Nikon didn't want to redesign the whole body, so solved the problem by constructing the mirror box to the same design but out of plastic instead of magnesium alloy. Plastic is more resilient than magnesium alloy. For the D850 the design was completely redone and the mirrorbox went back to magnesium alloy, presumably with more metal round the weak point.
I'm not sure that there's any good way of assessing 'build quality' without dismantling something and performing some stress testing on the parts. Certainly just talking about the choice of materials doesn't get close. For instance, Canon has maintained that magnesium alloy isn't suitable for camera chassis. They only use it for skins. The EOS1D has always had aluminium chassis (just like Nikons used to have) whilst everything else had plastic. I don't know so much about the mirrorless times. A big advantage mirrorless cameras have is no mirror box, which was always the weakest part of a DSLR - and at the same time the bit that needed to be strong to keep the lens and sensor in alignment.
Presence or not of 10-pin connector has no impact on build quality. For instance, the D500 has a 10-pin connector but has the same basic construction as the D5500 (and later), D750 (and later) and D7500. That is the Sereebo composite construction that appears to have been used for the Z8 (though being mirrorless, its structure will be completely different)