Preface: my knowledge here is on ARM, particularly baremetal, but also embedded Linux. No idea about Windows or x86.

Generally there’s a hardware watchdog implemented as a counter/timer in the processor. It can have a predefined or configurable period. It counts down, and if it times out then it initiates a hardware reset of the processor.

You can ensure your software/OS is always at least executing code by having a task (in-kernel on Linux, or an RTOS task, or just in your main event loop on baremetal) that resets that timer. Then, if your code stops resetting that timer, it expires and resets the processor.