Google announced the product reviews update on April 8, 2021 with a dedicated Search Central post, "What creators should know about Google's April 2021 product reviews update". The update was "designed to better reward" reviews that "share in-depth research, rather than thin content that simply summarizes a bunch of products", and it applied to English-language reviews. Google was explicit that this was a standalone update, not a core update, per Search Engine Land's coverage.
Google's guidance centered on the reviewer actually knowing the product: express expert knowledge, show what the product is like physically or how it is used, provide quantitative measurements, explain what sets it apart from competitors, cover comparable options, and discuss drawbacks as well as benefits. Reviews that merely restated manufacturer information were the kind of thin content the update was built to outrank.
Danny Sullivan confirmed on April 22, 2021 that the update was "effectively done" rolling out, 14 days after it began, while noting edge cases could linger. Data providers rated it a big update, though not core-update big. It proved to be the first of a series: Google's ranking updates history logs a December 2021 product reviews update as the follow-up, with further product reviews updates through 2022.