JOHN / JANE DOE are common generic names used in the USA primarily in legal actions if a person's true identity is not known, as well as corpses or hospital emergency room patients whose identity isn't known (in the case of infants, BABY DOE is used). If more than one person of the same family are involved, then names like JAMES DOE, JUDY DOE, etc. are used. An equivalent name used in court cases is RICHARD / JANE ROE, as in the landmark Supreme Court abortion case "Roe vs Wade."
Widely used in the US, this custom is rarely seen in other English-speaking countries, even though the JOHN DOE / RICHARD ROE custom dates back to England's King Edward III and a case called "Acts of Ejectment". This involved a hypothetical landowner named who leased land to another fictitious man named Roe who took the land as his own and then ejected/ evicted "poor John Doe." The terms DOE & ROE also refer to names given to members of the deer family ("doe" is a female deer, and "roe" is a type of deer found in England).
In state level court cases, female defendants with unknown identities are known as JANE DOE, and in federal cases as MARY MAJOR.
JOHN / JANE SMITH are generic names given to the "average person" . JOHN / JANE are common English first names, and SMITH is the most common last name.