Depends on the type of aircraft, on the Metro it's basically as you describe. There is a large rear cargo door and mounting points for nets so it's not a big deal. Some operators use a liner to protect the interior if the aircraft is used in both cargo and pax operations. Freight is VERY hard on interiors and it soon ends up looking scruffy inside.
On larger types a great deal more work is required, possibly comprising the installation of a cargo door and tracks for the containers to ride on.
As passenger appeal is of no consideration, older aircraft with a much lower capital cost can be used. However fuel consumption and maintenance costs are higher. Take offs and landings are usually at max weights. Utilisation and cycles are generally lower than pax operations. Reliability is very important as airfreight is more expensive than surface transport, your load is time critical. Start missing flights and things go haywire regarding connections, a package can easily change planes four times going across the country.
At the end of the day men in offices using spread sheets work it all out to make sure it pays.