From my point of view, the answer to your question depends on the content that you refer to by introduction and overview as well as the actual number of sub-sections to a respective level. I consider a sub-section to be an actual subsection as stated in your question as well as a section within a chapter.
The actual content should always be placed within the respective sub-section. Sectioning is supposed to help a reader finding content of interest fast.
Kind of Introduction
Depending what you aim to introduce (specialties of a method vs. a rather general field of study), the introduction should be placed within or outside of the sub-section.
Kind of Overview
Usually, I would expect an overview to be outside of the specific sub-sections of content. One may say, that an overview works out to be the same as an introduction to a rather general field of study, cf. Kind of Introduction.
Number of Sub-sections
The more different sub-sections you have with regard to a level, the more important I find an overview to interrelate these. Since I do not know which sub-sections to read a-priori, I expected an overview that sketches and relates the entire content right after the chapter/section heading. Depending on how interrelated the topics in your sub-sections actually are, this introduction/overview may be longer or shorter.