The goal is to write a thesis that links what you have learned during your degree, what you will learn in the books or journal articles on the subject you have chosen to write about, and what you will learn from your experience with professionals in the context of your courses or internship.
Even though it is written in the context of an internship, your thesis should not be the daily diary of an intern. In order to achieve the best results, imagine you are a new arrival in a team writing a detailed note about a subject of interest to the team in question.
Your thesis must identify an original angle and develop an argument. Its main role is to illustrate your ability to review and delineate a subject, to construct a relevant bibliography, to synthesize the important documents, to make use of both theoretical (studied in class) and practical knowledge (gained through your internship), and to think critically and to suggest directions for further research.
Le Fol, G. 2019, How to write a master thesis, Lecture notes. 12 pages