There are a number of issues associated with Power Delivery Networks.
If you have a parallel bus like a memory data bus, you could have all of the bits flipping from "FFF" to "000" at the same time. If that was a DDR2 64 bit data bus, the electrical problem is to turn on or off roughly 2 amps in 350 pico seconds. That means you are switching 2 Amps at a 3GHz rate. To do that you will need excellent bypassing.
If you only need a 20KHz audio amplifier supply, you only need bypassing up to about 30KHz. That is not to hard unless you have a very noise environment and you need a very quiet supply.
Then there is the world between these two end points in the power delivery universe.
I recommend watching the video out of PTU about power delivery section for general orientation.
Then I recommend watching the video about 6 layer boards from the Labs.
the video on 6 layer
Then I recommend watching the video about 8 layer boards from the Labs.
the video on 8 layer
Since everyone uses switching DC to DC power supplies and most people who build their own run into noise problems, I recommend viewing the Controlling Switcher Noise video