It should be noted that effective mid-November 2023, Google is switching to rolling updates to the algorithm that affects reviews. This is after several adjustments to the algorithm to improve the quality of the reviews, which appear in Google's search results. Also, the display of reviews are not a certainty for any website and may be triggered by Google's understanding of the intent of the search.
Reviews can be coded with schema as a JSON element or schema as properties of HTML tags. A Google review snippet may be provided for the vast majority of products, things, and places. They can be signal-person reviews or aggregated reviews.
product offers can be part of either the author review or aggregated reviews. However, reviews should not be self-serving, (Pepsi cola should not review Pepsi cola but a distributor or retailer can, so product offers can be part of a review).
The review snippet written by a person will include the author who wrote the review. E-E-A-T signals are present in these reviews.
The example shown on Google Search Central for review snippets; The E-E-A-T is to and from the Person of Bob Smith and Organization of Washington Times. However, without a URL or other unambiguous identification for Bob Smith, the review may not receive a benefit for the author. Washington Times being a Trademark is likely unambiguous enough.
For a product with an offer an aggregateRating is shown as the example on Google Search Central.