For any in-text citations with three or more authors, use the surname of the first author followed by ‘et al’ from the first citation - this is new in APA 7th edition.
For books with 21 or more authors, include the first 19 authors, then insert an ellipsis (three dots) and then add the final author (without an ampersand). See the Journal articles with 21 or more authors section to see how to do this.
Cite the authors in the order they appear on the title page of the book (do not alphabetise these yourself).
(Greig et al., 2013)
Grieg et al. (2013)
Greig, A., Taylor, J., & MacKay, T. (2013). Doing research with children: A practical guide.
Author Surname, Initial(s)., Author Surname, Initial(s)., & Author Surname, Initial(s). (Year).
Book title: Subtitle. Publisher. DOI (if there is one)