you replace the DVD burner with a second hard drive, inside a small caddy or enclosure that fills in the extra space.
the optical drive is connected to the logic board with a typical SATA connector, so you just have to make sure that you've got the right hard drive, and switch them out.
You forgo the ability to read and burn CDs and DVDs without an external USB drive, but most people don't use a lot of physical media these days, and the extra hard drive space/speed can be much more useful.
If they need to burn a DVD or something, they connect to the external burner, and burn away.