The most common option for drop-in coolers is cold-wall refrigeration. These models connect to a power source and use refrigerant to keep the inner walls of the food well chilled. Food pans are then set down into the well, where the cold walls help keep the temperatures in the food-safe region.
In addition to cold-wall cooling, some drop-in coolers also make use of a forced air flow over the tops of the pans. This airflow helps prevent the food on the tops of the pans from becoming too warm, creating a sort of shield between the cold food and the warmer air of the surrounding room. In these models, the food pans held by the unit are recessed a few inches below the outer rim of the well.
Another option you might consider is an ice-cooled drop-in cooler. These units are ideal for mobile operations and for setups placed somewhere where electricity might be difficult to connect to. These wells are recessed into the counter and have a drain that connects to an outgoing water line to make cleanup simple. Ice is placed into the well, then food pans or packaged products are nestled into or placed on top of the ice.