Monitoring Wi-Fi signals is, afaik, something that happens on the Wi-Fi chip itself, not the CPU.
While you’re correct that nothing stays truly idle, the modern design is that the main CPU really does stay largely idle because of the power costs involved and instead dedicated microprocessors absorb the load when possible.
While you’re correct that nothing stays truly idle, the modern design is that the main CPU really does stay largely idle because of the power costs involved and instead dedicated microprocessors absorb the load when possible.