Primary sources are original research articles (empirical research), meaning that they are written by the researchers who conducted the research. They are most often written once the research is complete or near completion.
The APA Dictionary of Psychology defines empirical as:
The APA describes peer review as the " process, in which the quality and potential contribution of each manuscript is evaluated by one's peers in the scientific community. . . . Toward the goal of impartiality, the majority of APA journals follow a masked review policy, in which authors' and reviewers' identities are concealed from each other. Reviewer identities are never shared unless the reviewer requests to sign their review." - https://www.apa.org/pubs/journals/resources/peer-review