The definitive source for guidelines using APA style is the Publication Manual of the American Psychological Association, frequently referred to as the "APA Manual." Most recently in its 7th edition, the manual will contain information for formatting papers, in-text citation examples, bibliography help, and sample papers, including information specific to student papers.
Publication Manual of the American Psychological Association by 
DOI is a digital object identifier
The DOI is a a web address linked to the publisher information of the article or book chapter. This is more stable than using a link from a database. A group of publishers, Publishers Linking International Association (PILA) formed the CrossRef depository as a collaborative linking system. To find more information go to https://www.apa.org/pubs/librarians/industry/crossref
To find a DOI for a publication you can use the CrossRef website: https://www.crossref.org/
A simple way to do so is to use the Search Metadata search and enter the title of the article. Then you can check for the matching item.
