ATC Altimeter Settings

When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC
region, they say something like, "Midway altimeter is xx.xx." I have
some questions.

1. So, do they pick some fix for the region and give everyone (those
below FL 180) the altimeter of that fix? Do they break up into
sub-regions and give everyone in that sub-region that same fix altimeter
setting?

2. At what point after departing an airport do you change altimeter
setting to the one supplied by ATC? Similarly, on approaching an
airport to land, at what point do you switch over to the altimeter
setting of the airport (from that given by ATC).

The reason I ask is that sometimes, the altimeter settings can be quite
different. Especially if there is VFR traffic around an airport, it
seems that there is some risk if everyone entering the pattern (or
approaching the pattern) does not share the same altimeter setting.

Just curious.

-Sami
N2057M, Piper Turbo Arrow III