Stupid FCF Question - Taxes

Can someone easily explain to me why, when you calculate FCFF from NI that you add back Int Exp after tax but Depr and Amort (NCC) fully? I get that paying interest provides a tax shield, but doesn’t Depr and Amort reduce your taxes too? I’m also missing why when you add back NCC to EBITDA after tax (now using EBITDA to calculate FCFF) you then use the tax shield amount. I re-read many times but still don’t get it… Thanks in advance!