Seven (sapta) Seers (rishi) who are the constellation ‚Great Bear‘.
Gautama, Gritsamada, Vishvamitra, Vamadeva, Atri, Bharadvaja, Vasishtha – the names vary in the scriptures.
The Vedas were revealed to them. Each Rishi is assigned a mandala (chapter) of the Rig Veda.
The Rishis were not humans (purusha), therefore the Vedas are apurusheya – not of human. What is written by human is purusheya.