What generates the clock signal in a fast CPU and how does it work?
Actually crystal oscillators can easily go up to 10's of MHz. Above that in most cases a PLL (Phase Locked Loop) is used, which is an oscillator that is not very accurate in itself, but can be tuned (its frequency can be adjusted somewhat). The frequency of this high-frequency oscillator is divided by a suitable factor (dividing a signal by a power of 2 is easy and totally accurate), and then compared to a let's say a 10 MHz oscillator. The comparison is used to adjust the high-frequency oscillator. Thus a high frequency is made with (almost) the accuracy of the lower frequency crystal oscillator.
In most cases, the circuitry to do all this is built into the processor chip. This is so it can be configured under software control, and routing such a high-frequency signal between chips is a nightmare.