Is it plagiarism to use other people's phrases / formulations / wordings? Is it plagiarism if I use other peoples formulations and phrases? What if I do so massively? Or let's say, the structure or rough form of their essays?

(I think it should be, but I would like to know what most people think, leaving aside that it might be difficult to discover it when somebody just copies someone very roughly in form)
There's no hard-and-fast rule that if you copy 6 words, that's okay but 10 is not but, copying the substance of someone else's work, making only cosmetic changes is called "close paraphrasing" and would likely be considered plagiarism even if the source is cited.  If it was, as you say, "massive" or if it omitted any citations to the original work, then, without doubt, that would absolutely be considered plagiarism.  As an instructor, I know that if I discovered it in something a student submitted to me, I would certainly report it as suspected academic misconduct and I would expect my report to be sustained.

Here's an article you may find helpful:  Successful vs. unsuccessful paraphrases