When they’re part of the pants, you have to wash your pants too frequently. Underwear should be removable, separate, and bike-ifying shorts or pants or knickers by stitching in the pad amounts to stitching in the underwear. You can buy separate padded undies (Andiamo! is one brand, and the exclamation make is built into the name).
The need for padding is another topic.
About Rivendell Bicycle Works
World-beatin' lugged steel bicycles & all the great stuff you'll need to build, accessorize & ride them. In Walnut Creek, California since 1994.