Maybe. Or maybe not. Airside transit does exist in China, the problem is not always. There simply isn't the level if internation-international transfers to justify the dedicated buses and screening point that you get at, say, LHR. Or as jiejie notes - the airports aren't really up to it operationally. But when they need to be though, they are - the process generally including an escort of some kind.

That's not what it means at all. As far as the 24 hour thing is concerned yes you can stay in the transit city for up to 24 hours - specifically though that part of the transit city that is the airside bit of the airport! It's referring to bog-standard airside transit (as noted above, when it needs to work it does).

As far as the 48 hour thing goes, then yes - it's only available at PVG and SHA, and only for certain nationalities. In terms of whether you can leave Shanghai during that 48 hour period - the wording doesn't specifically prohibit it, but neither does it specifically allow it. It's really down to the practical issue of whether not having a 'proper' visa will cause problems outside of Shanghai. In terms of travel then most probably not, in terms of hotels then possibly.