Yes that's a very good explanation by thylacine1975.
I think it's easiest to see capacitors as small energy reservoirs, that try to keep voltage at their terminals constant. When for example an IC switches outputs, there's short but large fluctuations in current consumption. The wiring /circuit board traces between power supply and that IC makes those current fluctuations cause
local voltage fluctuations (how much, depends on wiring resistance / induction, frequency & magnitude of the current fluctuations).
With no decoupling capacitors in place, those voltage fluctuations might be large. But with a decoupling capacitor in place, the capacitor either supplies (when voltage goes down) or consumes current (when voltage goes up), and by doing so causes the voltage fluctuations to become smaller.
A small capacitor (like those typical 100 nF ceramic) reponds very fast, but has small capacity, so it's good at supplying current for very short periods (like the current fluctuations you get when fast digital logic switches outputs). But because of that high-frequency behaviour, it needs to be very close to the point where those current fluctuations originate, to do its job effectively. Like directly across GND/Vcc supply pins of that logic IC.
A large capacitor (like the electrolytic ones you find inside power supplies) has much larger capacity, but in comparison lousy high-frequency response. So it would do nothing to counter high-frequency current fluctuations. That's why you tend to find these capacitors at board power supply entry points, or scattered in a few places across a board - they deal with low-frequency, high-current power fluctuations.
So effective decoupling uses as combination of different capacitor types in parallel, location & types depending on application. As for exact numbers, you'd have to know frequency & magnitude of the current transients, the exact characteristics of board traces, impedance vs. frequency of the capacitors used, etc etc. Not impossible, but in a way a kind of black art... ;-)
Most papers use charge sharing, however, make the assumption that all the immediate current requirements come from the decap with no current at all coming from the voltage source. Is this a fair assumption?
Yes, for high-frequency current transients, that sounds a reasonable assumption. Hence the problems you might have when you have such high-frequency current transients, but no decoupling capacitors in place. Although small in size, their
local effect can be extremely important.