It's my understanding that modern airliners are pressurized using bleed air from the compressor stage. But how did piston-engined aircraft like the DC-6 get pressurized? I couldn't find any answers anywhere on the internet and I don't know how the pressurization worked
The DC-6 was equipped with two "cabin superchargers." A supercharger is an air compressor that is driven by the engine's crankshaft. A typical use of superchargers is to provide compressed air to the engine's intake to provide greater engine power, but these "cabin superchargers" instead fed into the cabin to provide the necessary air to maintain the cabin pressure.
A cabin heater provides hot air if needed, some of the air from the superchargers is routed to an "aftercooler" to cool it down, and some small amount of cold air is available from a "cooling turbine." These different sources of air can be mixed together to provide a comfortable cabin temperature.