I get the pad from the seniors' design which contain all the metal from bottom to the top.
However I did not get any explanation so I also need some advices.

Thanks

As bernd indicates, the details depends on your foundry.

But RF pads benefit from being top metal only if it is mechanically robust enough for the bonding process. I have used technology where this was allowed... others where it is not.

I have also seen a trend for circuitry (mostly ESD or IO drivers) to be incorporated beneath pads, in which case the metal layers which constitute the pads are the higher levels only.