Is it plagiarism if peer reviewers use sentences from submitted manuscripts? I have a question about plagiarism.

If a peer reviewer writes a negative review of a paper and rejects it, so that he or she can use the idea of the paper in their own work is that considered plagiarism?

Similarly, if a peer reviewer lifts sentences from a submitted, but rejected manuscript into their own paper is that considered plagiarism?

And if you have a strong suspicion either of those two have happened (e.g. you see a publication appear after a while with exactly the same phrasing as your submitted manuscript), is there anything you can do about it?
The answer to your first two questions are easy: Yes! It is considered plagiarism to take other people's ideas and represent them as your own, without giving proper credit. The same holds for sentences in a paper, no matter if it has already been published.

Your last question is more difficult to answer, as the burden of proof essentially lies with the person being plagiarized. Since the paper have already been submitted (though rejected), there should be a date stamped version of it somewhere in the journal repository. This is, however, not the best way to go, as the supposed culprit(s) can claim that they have never seen this paper - it is, after all, not in the public domain. For this (and other) reason I would always recommend authors to submit preprints to a service like arXiv, to have a valid, time-stamped proof in the public domain.

In the concrete case I would recommend contacting the editor of the journal where the new paper has been published, if adequate proof can be gathered.