Vyasadev wrote the four veda.
Then the Upanishads developed around it as comments develop around a blog post.
To explain things more clearly to the common audience of simple people, Vyasadeva wrote the itihasa such as mahabharat and ramayana and the puranas
To clarify the morals and philosophical principles behind the stories VedaVyasa wrote the Vedanta Sutra – the Conclusive Thread Woven throughout the Collective Veda.
After this monumental effort he still felt unsatisfied, like his work was incomplete. He consulted his guru, Narada Muni who told him, “yes you have ignored the central mystical purpose of life! You have neglected to point souls explicitly to their eternal devotional relationships with Godhead. Therefore you feel incomplete despite all the work you did. So write a commentary on the Vedanta Sutra explaining this.”
So Vyas wrote the Bhagavat Purana.