There are generally many APs for a single ETF or family of ETFs. Historically they have been the ETF desks of large banks[0], more recently marketing making/trading firms have build out ETF AP capabilities[1].

[0] https://www.investopedia.com/terms/a/authorizedparticipant.a... [1] https://www.janestreet.com/what-we-do/etfs/