That's right. Because bread is uncountable, you use "loaf of bread" to describe a block of bread that hasn't been cut. A slice of bread is what you make by slicing the bread. For example, I might make 16 slices of bread from a loaf of bread. A sandwich is made from two slices of bread.

The loaf of bread is the full portion of bread. When you cut the bread you making slices. The example you made about cutting the bread and the sandwich is correct.

Maybe you'd like to know the phrase, "The best thing since sliced bread" (Variant: "The greatest thing since sliced bread.") If something is the best thing since sliced bread, it means that it is excellent; wonderful." Examples: "In my opinion, the iphone is the best thing since sliced bread." (= In my opinion, the iphone is really an excellent product.)

yes, whole = loaf, cut pieces = slices