Short answers, pulled from the story.
The Vedas were composed in Vedic Sanskrit between 1500 and 900 BCE and were not written down until after 500 BCE. The Rigveda was composed between 1500 and 1200 BCE while the Atharvaveda was compiled last around 900 BCE.
The Vedas were preserved through a rigorous memory culture using eleven distinct modes of recitation including the mesh recitation method. This system required every two adjacent words to be recited in original order, then reversed, and then original order again to ensure no word was ever lost or altered.
The four canonical divisions of the Vedas are the Rigveda, Yajurveda, Samaveda, and Atharvaveda. The Rigveda contains 1,028 hymns, the Yajurveda consists of prose mantras for rituals, the Samaveda is the repertoire of singer priests, and the Atharvaveda contains 760 hymns with magical formulas and charms.
The central theme of the Upanishads is the identity of Atman and Brahman as the fundamental principle which shapes the world. These texts were composed between 800 BCE and the end of the Vedic period and reflect philosophical speculations on the connection between human organisms and cosmic realities.
The Rigveda that survives in modern times is in only one extremely well-preserved school of Shakalya from a region called Videha in modern north Bihar south of Nepal. The Rigveda was transmitted in various schools or shakhas each representing an ancient community of a particular area or kingdom.