[quote=frankj1]and the rest is the beans one buys?[/quote]
A little bit, but it has more to do with preparation. Age more than varietal or roast, assuming it's not burnt.
Water that is too hot will scorch the coffee and, higher temp water will pull out more tannins (what is responsible for the bitterness). An extreme example is cold brew, which has practically no tannins in it, no matter what coffee you use.
Grinding too fine will also allow too much contact area and as such result in higher concentration of tannins. This gets exacerbated with water that is too hot.
Yes, some beans are just ****, but even the mass market stuff (Foldgers, Maxwell House, Yuban, Chase & Sandborn, etc) that isn't highly regarded, improves drastically when brewed properly in something like an aeropress.
Poor water quality creates off flavors, not necessarily bitterness.