Suppose I get a detailed review and implement many things in response. Where do I reply to say what I did? Presumably, reviewers want to know that their suggestions were useful.
Presumably, reviewers want to know that their suggestions were useful.
Indeed! As with all Stack Exchange sites, useful answers receive upvotes from readers. If you haven't done that already, give that review your upvote, and mark your question as answered by giving a checkmark to that awesome answer. If you have >50 rep and more to say than just "Thanks!" or "Awesome, +1!", you can leave a comment too, though these comments often get flagged (and soon removed) as too chatty or obsolete.
Sometimes a question doesn't get many views, and still gets awesome answers, but then these answers don't get as many views as some others (say, on some more popular question) do, and you feel like your petty +25 doesn't do justice to the awesome answer you got. Browsing to the answerer's profile to upvote all their answers is not the solution - serial voting gets automatically reversed, so voting for users isn't a good idea.
If you have the rep, you can award a bounty to a specific answer, specifically to reward the reviewer with additional rep from your own stash.
Code is never really flawless though, so you might also want to follow-up with a new question (it's rather commonly done here) - see this Help Center page for all the relevant information.