You always run your study based on a subset of the entire population, you intent to generalize on. External validity refers to the extent to which you really could do so. This in turn depends on the sampling methods you where using.
Writing a thesis is often a challenging and complex enterprise. This episode helps you to understand what will go on inside you and between you and your supervisor.
All slides to the entire series can be downloaded for free here: https://armintrost.de/en/professor/digital/social-research-methods/
Writing an academic report or article mainly follows a specific structure, format and style since academic papers differ from all other media most people are familiar with (e.g. news-paper article)