I'm not quite sure how people get the 100% flat ones. Might be a different yeast...?
I suspect it's simply a case of allowing the fermentation to go to completion in a vented vessel. That way all the CO2
comes out, so there's no fizz. "Real" or "traditional" cider is not pasteurised, as far as I know. I think most mass-produced ciders are artificially carbonated.
Also, some thoughts on terminology.
In the UK, "cider" always means fermented
fruit juice - almost always apples. Sometimes it's also used for fermented pear juice (i.e. "pear cider"), but there is in fact an older word, "perry", for this, albeit little used now.
In the US, as I understand it, "cider" still refers to apple juice, but doesn't necessarily imply it's been fermented.