You can buy a complete external cooling system that uses tubes to get liquid into and out of the water blocks in the case (on the CPU, for example). What that means is, except for the water block(s), this external water cooling accessory has everything else inside of it. The unit would contain the pump and a reservoir as well as the radiator. Depending on the sophistication, these may have temperature indicators. Attaching the water block to the target chip and attaching the inlet and outlet tubes are all that is required to get these kinds of systems going.
A third approach to a PC water cooling system would be the DIY approach. You buy all the accessories individually and install the system yourself. You will need to find ways to install each of these components inside your PC case and make sure they work effectively. Particularly, the radiator must be able to cool the circulating water properly.
It is possible to cool multiple components inside the PC. Water tubes will have to create the cooling loops through the multiple water blocks, of course. Tube splitters are available that let you create these additional paths quite easily.