> A translation tool like DeepL is presumably trained on a huge amount of 'other people's work'. Is copying its result verbatim into your own work also plagiarism then?
Yes, if you present yourself as its author.
So let's say you are not a native English speaker and write a passage of your paper in your native language, then let DeepL translate this and paste the result into your paper, without a note or citation. Is that plagiarism?