In OJS 3.4, the entire process is handled through a single main plugin, the Crossref Manager Plugin, with further configuration under Distribution > DOI.
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.3/
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/
Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/ 

Image source: https://www.crossref.org/documentation/register-maintain-records/ojs-plugin/ojs-3.4/