Permited peer assistance in PhD and MSc thesis Where I live, it's quite common for MSc students to seek help from "professionals" in this field. What they do varies from simple peer-review-like service and in the worst case they provide a full thesis with tuition i.e. spoon feeding.

It's amazing how many people teachers and teaching assistants are (unofficially) accused of getting this type of degrees. But this accusation in most of the time actually holds due to obvious lack of fundamental comprehension of the topics they are specialized in.

However:


Peer review is quite common and a prerequisite for papers to get published in a respected journal. 
It's typical for a professor to have one or more teaching assistant to help him doing some part of the work for example some CS professors delegate the proof of concepts implementation to a teaching assistant.


Besides the two examples above what else is allowed and considered an acceptable academic activity by a peer:


Translation for thesis written in another language to English which is the required thesis language in that university.
Writing a proof of concept off the record i.e. without citing the PoC creator in the thesis officially.
Writing the (Related Work / Literature Review) given the student claims full knowledge of the papers but having no time for writing it.
Does the above changes when it's done as a service i.e. paid, instead free peer assistant?


Note 1: I do have my own view for this but I would like to hear yours. Also I will mention my view later in a answer for this question.

Note 2: I'm concerned with the STEM fields if that makes a difference.
In my opinion, none of your numbered items (1-4) is acceptable. As for the two bullet points above, they must be carefully circumscribed:


You can of course get comments on your manuscript from your peers. This should not include solving your problems for you or writing parts of the work.
Some technical work may be performed by someone else: chemical synthesis of a compound you are using, specialized analysis of some samples etc. Each instance should be clearly specified: in the text, in the legend of the figures obtained this way etc. The reader needs to see clearly the distinction between your effort and that of other parties.


All this is codified in the saying: "Credit where credit is due". However, this principle is applied slightly differently to a thesis and to a journal paper:


A thesis demonstrates the ability of its author, who is assumed to have done all the work contained in it. That is why any external contribution should be indicated.
A paper's main goal is to present a scientific result. It is therefore less crucial to know "who did what", although some journals (e.g. PNAS, PLOS One) do require it.