What's the name of the piece connecting metal and plastic pipe under the basin? Or what is it commonly known as? (easy to buy)

First, the drain pipe under the washbasin is customarily called the sewer pipe, and the scientific name is "sewer". 2. Installation steps of the drainer: 1. First, take out the drainer, disassemble the fixing piece and flange at the lower end of the washbasin drainer, and then fasten the flange of the washbasin drain pipe under the washbasin. Be sure to tighten the flange and put the washbasin on the countertop. ...