Google notices a series of review sites that it then uses to help support its ads and also consumers when making an informed decision about a product or service.
The reviews will in most cases be pulled into Google ads and displayed as an aggregate Star rating. You will also be able to click the ratings to see all the reviews. e.g. https://www.google.co.uk/shopping/seller?q=caravanguard.co.uk
Your start ratings will appear in your google ads if you have more than 30 reviews over 3 stars in the previous 12 months.
As for selecting a site to push your reviews to I would suggest you read this post.
1. Click on the + We base seller ratings on a number of sources including:
2. Then Click + We feature reviews via feeds from these independent review websites.
It lists all those who are in effect Google Partners (Feefo, TrustPilot, Revoo and many more) these are probably best sites to use if you want a connection. However, do not discount the use of Facebook, G+ for further social proof and trust factors that consumer now crave when making in informed desicion about a purchase.
Finally as a note - if you do not use any of the Google Partner sites, make sure you bring some reviews to hold on your own site, maybe a simply review collection form, You can then use the schema for rating to display your aggregate ratings in your organic listings, this helps them to stand out from the rest.
See here for Aggregate Ratings Schema and here for individual ratings.
Hope this helps.
Tim
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