What happens to the reviews that people write for journal articles after they're sent back to the author?
In my experience, the contents of the comments of other referee reports are only made indirectly available. Since the authors are normally expected to provide a response to the reviews, the relevant criticisms and comments of the other referees are typically mentioned or discussed in that document. Outside of that, however, there's often little direct sharing of referee reports. None of the eight or nine journals for which I've reviewed (physics, chemistry, chemical engineering) have allowed me to see directly the reviews submitted by the other referees.

At any rate, the results are almost always kept confidential, unless it is an "open" referee process by design. (There are a few journals now that make the refereeing process a part of the publication record for a given paper; an example is The Cryosphere.)