I believe that the certificates are just automatically generated once you complete a course, if you signed up for the paid version of it. Note that it may take up to a couple of days for the process to happen. This is done by Coursera, not by DeepLearning.AI. Meaning that the certificate will say DeepLearning and give the name of the course, but it is generated by Coursera.
You can try checking with the Coursera Help Center for more information.