how know z- . index element code example

Example 1: css z-index

#nav {
  position: fixed;
  ...
  z-index: 2;
}
#main {
  position: absolute;
  ...
  z-index: 1;
}

Example 2: what is a z index in css

Definition and Usage. The z-index property specifies the stack order of an element. An element with greater stack order is always in front of an element with a lower stack order. Note: z-index only works on positioned elements (position: absolute, position: relative, position: fixed, or position: sticky).
Default value: auto