When you create a widget, the default location mode is Automatic, which means that Fera will scan and choose the best location for your widget.
When adding a Product Reviews widget, you cannot explicitly add reviews to one product page and not others.
If you have a many product reviews for Product 1 but none for Product 2.
What you can do instead of having unrelated product reviews on a page, you can make the Product Reviews Widget not show any reviews should a product not have any.
Another alternative could be to use a Customer Photos & Videos Widget when you have no reviews for certain products.