Musicians in the western tradition, prior to recording, was disseminated mostly aurally, and at a much later point a mixture of aurally and through written music.

Back in the day of chant, for example, the singers would be expected to memorize the piece of music which would be transmitted orally. The written component, if it existed, would either be with neumes (little markings over the text) or maybe notated music.

However, this sort of notation isn’t the most accurate, and to learn the “proper way” to sing the chant, it would simply be via learning it from others.

As time goes on, written music tends to become a bit more important for the learning of music, and a lot more precise. Beethoven’s music can be reasonably interpreted just from the urtext, while with Gregorian chant we have vague ideas on what the composer may have wanted/imagined it.

Troubadours would also have learned/taught their music aurally to minstrels and other performers.

This sort of learning does lend itself to warping the musical content overtime.

Person->person Aural practice is actually pretty common today! Many music students reference recordings, or their teachers will show them the “proper” way to play/sing things.

Tl;dr you learned it from other people. However, your song may have warped from its original form.