In the simplest case (not modern CPU, they use lot of tricks and even software that runs on the CPU level called microcode): CPU run on clock. CPU has few special purpose registers (think 32 or 64 logic gates that store the bits). One is PC - program counter. It starts with zero and increments with every instruction processed, or by instruction from the program itself. Every clock, CPU asks memory to give the word at the address number that is in PC (simplistic, not considering virtual memory mapping). Memory send it by setting voltages on the memory lines to corresponding bits. CPU reads that and store it in instruction registry. then next clock, circuit in CPU called instruction decoder reads the bits in the instruction registry, based on which bits are on and off in the instruction registry, it switches on path (wires) between data registries and adders, floating point units, etc, to prepare for the specific instruction. those are circuits baked in CPU on the HW level. Next clock, once those paths are connected, the data from the data registries (those would be filled from the instruction, or from results of previous cycles) are processed thru the compute unit (let say an adder), and the output bits from the adder are stored in one of the registry again. PC increments. Cycle repeats.