In a nutshell
The datapath is the chain of hardware an instruction physically travels through. The program counter points into instruction memory to fetch the instruction; the register file reads its operands; the ALU computes; data memory is read or written for loads and stores; and the result is written back into a register. The control unit sets the signals — including the mux selects — that route data correctly, and a single-cycle datapath does the whole journey in one clock tick.