Given that the cabin is pressurized, I assumed that means sealed, how basically do airliners take in air (ie, for ongoing breathing, etc) when high?
What is the valve?-like mechanism that does this, or .. ?
Or are they not really sealed that tight, and they are pressurized by just continuously pumping in a bit of air?
(I think the cargo area even is pressurized on some/all? airliners these days, is this part of the equation?)
The jet engine compressor section produces an ample quantity of high pressure air. When we divert some of that air (rather than letting it continue to the combustor and turbine), we call it 'bleed air'.
Some bleed air is used in the engine itself -- primarially for cooling the turbine.
At startup, bleed air from one engine is used to start the other engine. An APU (small jet engine in the tail, auxiliary power unit) produces compressed air to start the first engine.
In flight, bleed air is used for the ECS system -- environmental control system.
Cabins are pressurised in the sense that they are designed to keep the pressure inside them to a value higher enough to let an averaged human body breathe without difficulty.
Anyway they are not 100% airtight for one simple reason: let's say that on ground the pressure is 100 (we don't care about the units of measurement right now). At the standard altitude at which a jetliner flies, the pressure is 20. 20 is a way too small of a value for a human body to be able to breath without an oxygen mask therefore we have to isolate the fuselage and let the pressure inside stay higher than 20. Ideally we could perfectly isolate the fuselage and keep the pressure at 100 but this would be critical from a structural point of view: the fuselage would be "pushed outside" by the inside pressure of 100 and "pushed inside" by the outside atmospheric pressures of 20 i.e. the fuselage should support a net pressure of 100-20=80. This would be totally possible, but the structure would be so strong and heavy that the airplane could simply not fly.
As a compromise we actually let air flow out when climbing so that, when the cruise altitude has been reached, the pressure inside the cabin is 60: this value is lower than the 100 on ground but it's still breathable and, most importantly, gives a net pressure acting on the fuselage of 60-20=40 i.e. half of before. This net pressure allows the fuselage to be built light enough to be safely brought in the sky.
Obviously now the problem arises that we cannot simply open the window to change air due to the difference in inside vs. outside pressure: a system working with pressurised air has to be used as nicely explained by @RobMcDonald in his answer.
They are really not sealed that tight. There can be leaky areas, and there is one or more outflow valves that intentionally leaks air.
To get new air, a valve is not enough. The pressure outside is lower, so an open valve lets air out rather than bringing in new air. Instead, a compressor of some kind is used. Turbine engines compress air anyway as part of their operation, so many larger airplanes redirect air from the compressor of the engine to use for this purpose. But there are also airplanes with a dedicated air compressor for cabin pressurization.
