The Avesta that survives is a fragment of what once existed, and the tradition itself says so. Later Zoroastrian literature describes a much larger collection in many books, of which what remains is the liturgical core, preserved because it was recited.
Within it the Gathas stand apart. They are in an older dialect, they are metrically distinct, and they are attributed to Zarathushtra himself. They are also extremely difficult: the language is archaic, the poetry is compressed, and translations diverge to a degree that has real consequences for what the religion is taken to have originally taught.
The Avestan texts were transmitted orally for a very long time before an alphabet was devised specifically to record their sounds, and the manuscripts that carry them are comparatively recent. Priestly transmission preserved pronunciation with great fidelity across centuries in which understanding of the language itself was fading.
The Middle Persian literature of the ninth century and after, the Bundahishn among it, supplies systematic cosmology and law. It is invaluable and it is late, written under Islamic rule by a community defending and consolidating itself, and it should not be read straight back onto the Gathas.
Outside sources add a further layer: Greek and Roman writers on the magi, Achaemenid royal inscriptions that invoke Ahura Mazda without mentioning Zarathushtra, and the practice of the living community in Iran and among the Parsis of India.