Not really though and that’s actually a pretty common misconception.
If you have two elements (containing other elements) and one is relative and the other is fixed (or any other combo
really) their z-indexes are compared. A high z-index in a container’s child is not overiding the parent’s z-index.
So in your situation the 2000 was internal in the nav section but the nav section itself was lower that 1000.
I hope I cleared this for you Nicola! 