I am trying to extract the z coordinate value from the derived attributes, similar to How to re-calculate area, perim, p2a,width and length in QGIS? but I cannot see a dedicated operator for finding the z coordinate like when trying to find area within the field calculator.
You could try How to „extract“ z-values ? to open the attribute-table, go directly into the field-calculator. Within the geometry-functions you will find z
Just calculate z on the geometry: