Yes, you must install it first. Here is the link to download it.
Yes, you must enable it.
Please read following article.
Product reviews and ratings. Product reviews is one of the best forms of the marketing. Asking for feedback on products gives customers a voice, at the same time lettings potential customers know what other people think about your products. nopCommerce also supports the review helpfulness feature (“Was this review helpful to you?”)
Should be installed and enabled first.
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