When a submitted article is invited for full peer review, it is assigned to a review editor. The peer reviewers, all active scientists, share and discuss their comments with each other. The review editor uses the comments to write one letter back to the author with instructions.
eLife describes the process:
Reviewers get together online to discuss their recommendations – communicating openly with one another before a decision is reached, refining their feedback, and working to provide clear and concise guidance to authors. If the work needs essential revision before it can be published, the reviewing editor incorporates those requirements into a single set of instructions for the author to move ahead to the next step. We aim to deliver decisions after peer review within four weeks.Once the final article is published -- this Decision Letter is part of the material openly accessible with the article. The Author's response to the letter is also published, along with reader comments.
This gives the review process unprecedented transparency.