Is it allowed to publish an explanation of someone's thesis? Last spring, I did a research project where I tried to develop code, modelling a physical system using a method developed by a researcher and presented as his PhD. thesis.
Now I have completed my work and am required to present documentation of the theory used and the performance of code. Now, while I was explaining that paper, I did detailed derivations of all the steps taken which had been omitted from papers for brevity. These derivations I have put in the manual as well so that any other undergraduate trying to work with same paper can quickly get started.
However, is it plagiarism to upload such a manual on my homepage and GitHub repository where someone else did the hard work of deriving concepts in the thesis and I just present proof of intermediate steps?
I am unsure if this is a right practice even if I state that I am only explaining his derivation.
What else can be done to mitigate the copyright infringement? One option, I think, is to submit the report to my supervisor and he will personally distribute it to students in his research group who he thinks will benefit from it.
Properly credited, posting a piece that fills in all the details of a published paper (or thesis) should be fine. Indicate clearly that  the ideas are not your own - that you are just hoping to help others read the original work.
You should probably write the person who did the original research, thanking them for their theorems and telling them what you hope to do with your work filling in details. They will probably be delighted that someone has read their thesis.