A softener loop is preplumbed as the house is built. It provides a place where the plumbing is exposed so you can cut into it and run your inlet and outlet lines from/to it. In some cases, it should always be done but isn't, there is also a drain connection preplumbed.
If there is no loop, and the water service line plumbing is exposed, you install a softener the same way as with a loop and run the drain line to a drain line, sump pump or floor drain etc..
If the plumbing is not exposed, then you have to find where the service line comes into the building and that usually requires opening walls where the line most likely comes in.
Usually the only houses that have a loop are those on a slab and the loop is in the garage; sometimes by the water heater where the inside water shutoff valve is. Outside faucets and any irrigation is usually plumbed ahead of the loop so they don't get softened water. If a house isn't preplumbed, the outside faucets probably will be plumbed after the location the softener is installed in.
You need a water test to know how much hardness etc. is in the water, or look at the water company's web site if you don't have your own well.
Quality Water Associates