I'm not aware of any way that this can be done - there is no way to tie a review back to a specific user on a shop based on the review itself on the storefront, at least when using Shopify's free product reviews app.
There's also an additional layer of complexity here in the vast number of review apps that are available, and how they all can and likely do post and store their own reviews differently. Because of this, it would be unlikely, or at the very least extremely difficult to find a catch-all way to retrieve all reviews on a store.
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