How our ancient rishis preserved the Vedas flawlessly
The ancient sacred scriptures of Hinduism were not found engraved on a native rock, embossed on cast metal, papyrus material, or any birch bark manuscripts. In Hinduism, we do not have any founder or any emergence dates like with the Semitic religions or even Indic offshoots like Buddhism or Jainism. We also do not know the any event or how far we need to go to find traces of Hinduism.
What we find in Hinduism is a huge volume of ancient sacred texts preserved by great seers for several millenniums. These rishis passed down their vast knowledge and our heritage from century to century from their deep memories. This knowledge was transferred through many ages by the teachers to their disciples, without ever writing them down and was later organised by Veda Vyasa rishi.
The word Veda (वेद) is made from the word ‘Vid’ which means "knowledge" in Sanskrit are the oldest sacred text available. This is the reason why Vedas as termed as ‘Anantha vai Vedaah’ (अनंता वै वेदा:), i.e. Vedas are infinite. All other scriptures in Hinduism are derived or inherited from the Vedas. Vedas are also called as Anaadi (अनादि), one which has no beggining or end and hence eternal. Vedas are the primary and authoritative source of knowledge and are also known as Shruti literature, one "which is heard and should be remembered" by the rishis from god.
As the bedrock of Hinduism, we all know the authoritative division of the Vedas is fourfold: Rigveda, Samaveda, Yajurveda, and Atharvaveda. This sacred knowledge is passed through generations, ages, millenniums without any slightest alteration and adjustment in it. But questions come how it is possible even?