According to the APA's web site, 

A DOI is a unique alphanumeric string that identifies content and provides a persistent link to its location on the internet. DOIs can be found in database records and the reference lists of published works.

• The DOI is often located on the first page of the electronic journal article, near the copyright notice. The DOI can also often be found on the database landing page for the article.
• The main reason for DOI numbers is to provide a stable link to electronic articles accessible on the web.  The system is set up so that the DOI numbers will not change, and the articles and other digital documents that use them should remain accessible by using the DOI number to access them.
• However, some articles do not have DOI numbers, especially those that were published before the 2000s, and the APA Manual gives you alternative options for articles in that situation.  They provide some clarification about when to use a DOI number and when to use a URL in a citation to an electronic journal article from a database in a flowchart from the APA blog.  Ultimately, though, your instructor is the final arbiter on how you cite journal articles without DOI numbers.
• There are two formats for DOI numbers; one that is not a link, and one that is a clickable link.  See the two examples below:
  • Example 1: A typical DOI number in non-link format:  10.1007/s10699-014-9361-3
  • Example 2: A typical DOI number in link format (with the https:// included):  https://doi:10.1080/1057610X.2013.832117

Note:  In APA style (7th edition), the DOI number is required to be represented as a clickable link.  More recent DOI numbers are typically already in clickable link format (such as in Example 2 above).  If, however,  the DOI you are citing is not in link format (as in Example 1 above), you must convert it to a link by doing the following (as shown below using the DOI in Example 1 above):

To convert a non-link DOI to a clickable link:

• Drop the "doi:" and add https://doi.org/ (illustrated in blue below) directly in front of the actual DOI number (with no spaces):  https://doi.org/10.1007/s106.....