Where iz the z-index placed

Answer 1

you can use from z-index for a tag placed on another tag.
z-index:2; behind z-index:3;

Answer 2

But be careful z-index works only with properly used position property.

Answer 3

The Z-Index is the order in which elements appear in the third dimension(sometimes referred to as height), an element must have a position on either the x-axis or the y-axis(or use position:relative) for a value in the z-index to have an effect.

for example.
I have two elements which use position absolute. element a and element b. For some reason, element b is on top of element a.

Give element B a z-index of 1 and element A a z-index of 2 and element A will now be on top of element B.

A z-index just gives order to things which are stacked.