Modern computers are designed to idle at 0% then temporarily boost up when you have work to do. Then once the task is done, they can drop back to idle and cool down again.
One of the older ways (in x86 side) to do this was to invoke the HLT instruction https://en.wikipedia.org/wiki/HLT_(x86_instruction) : you stop the processor, and then the processor wakes up when an interrupt wakes it up. An interrupt might come from the sound card, network card, keyboard, GPU, timer (e.g. 100 times a second to schedule an another process, if some process exists that is waiting for CPU), and during the time you wait for the interrupt to happen you just do nothing, thus saving energy.
I suspect things are more complicated in the world of multiple CPUs.
However for a CPU with multiple cores, each running at 2+ GHz, there is enough room for idling while seeming active.
> Specify a tolerance for the accuracy of when your timers fire. The system will use this flexibility to shift the execution of timers by small amounts of time—within their tolerances—so that multiple timers can be executed at the same time. Using this approach dramatically increases the amount of time that the processor spends idling…[1]
[0] https://arstechnica.com/gadgets/2013/06/how-os-x-mavericks-w...
[1] https://developer.apple.com/library/archive/documentation/Pe...
The p cores can be activated and deactivated very quickly, on the order of microseconds IIRC, which means the processor always "feels" fast while still conserving battery life.
A modern CPU also has multiple cores not all of which may be needed, and will be supported by hardware that can do lots of tasks.
For example, sending out an audio signal isn’t typically done by the main CPU. It tells some hardware to send a buffer of data at some frequency, then prepares the next buffer, and can then sleep or do other stuff until it has to send the new buffer.