Johnson is about as straightforward as surnames get: it began as “John’s son.” John itself was a runaway favorite personal name in medieval England after the Norman period, so thousands of unrelated boys grew up as John’s son and later locked that label in as a hereditary surname.
In the United States the spelling also scooped up immigrants whose names meant the same thing in other languages—Swedish Johansson, Norwegian Johansen, and similar forms—because clerks and neighbors heard “Johnson.” That does not change the core story: a father’s given name, John, turned into a family badge. Unlike smith-names, it does not describe a job; it points to a parent.