Question

According to whom vedas are not 'apaurusheya'

Answer 1

Apaurusheya means not belonging to a person, or for a book we can without any author. We can also say that Apaurusheya means eternal and supernatural.

The rishis and sages only discovered the Vedas and there is no evidence that Vedas were written by someone. 

Answer 2

Apaurusheya indicates "not of a man", implying "superhuman", or "impersonal, authorless", is a connection which is utilized to represent the Vedas, as the original manuscript in Hinduism. These schools believe the Vedas as svatah pramana.

As per Purvamimamsa, Vedas was described as apaurusheya because the phonemes in Vedas have survived perpetually. The most common explanation is that Vedas emerged from divine origin and was first exposed to Rishis.