they vs who

they

pron
  • There (especially as an expletive subject of be). 

  • People; some people; people in general; someone, excluding the speaker. 

  • A single person, previously mentioned, but typically not if previously named and identified as male or female, especially if of unknown or (since 21st century) non-binary gender. 

  • A group of entities previously mentioned. 

det
  • Their. 

  • The, those. 

who

pron
  • With antecedent as subject. 

  • What person or people; which person or people; asks for the identity of someone; used in a direct or indirect question. 

  • With antecedent as object: whom. 

  • Whoever, he who, they who. 

det
  • whose 

noun
  • A person under discussion; a question of which person. 

How often have the words they and who occurred in a corpus of books? (source: Google Ngram Viewer )