It’s “folk music”. This means that most tunes have no known composers and are part of the traditional repertoire. Tunes in tradition are often passed from fiddler to fiddler down the generations, and are frequently known as “the version after so-and-so” (or after his son, or his uncle, or his teacher, or his grandmother’s singing, and so forth). Sometimes the tune comes with a name, and sometimes not. Sometimes multiple names apply to the same tune, reflecting different descent or the whimsy of a more recent player.
Some musicians made collections of tunes that they learned, or that they heard, or that they composed. Sometimes we know the collector’s name, and sometimes not.
Formal music education was not unknown. Musicians came from various levels of society. Some were well-traveled, and some went into the army and came back with popular tunes and dances. Some tunes and harmony parts show a familiarity with classical music practices, while others are modal or pre-Baroque in feel.
There are famous musicians and musician families that go back well into the 1700s. Sometimes we know who individual composers are in the older generations. Some musicians are so well-respected that many more tunes are attributed to them than is entirely likely.
Tunes travel and evolve. Traditional tunes change over time. There is no one “right” version. Memory is imperfect, regional styles change, individual musicians settle in a new place or pick up new favorites from elsewhere and popularize them, and art tunes pass into the popular repertoire. Individual musicians within the tradition may have favorite variants, preferred keys, and idiosyncratic rhythmic emphases. The parts of a tune can separate and recombine, or take on entirely independent lives. Sources vary: two of the most widely spread tunes are “Vårvindar Friska”, a simple folk song, and "La folia", a classical theme that has been popular since the Renaissance.
It’s a living tradition. These tunes are still being played and musicians are still writing new ones in the old styles. People still get married and need processionals and dances for the wedding. Children are still being born and getting baptism tunes. When a tune’s composer is known, it is acknowledged in the title or in the score (e.g., “Polska av Pers Hans” means "composed by", vs "Polska efter Pers Hans", which means "learned from his tradition"). For such tunes there is a “correct” version, but even these tunes are played with variation by their composers and will evolve over time as the tradition works upon them.