The manifest (at least how I am using the term) is whatever metadata a file has, and the format and location of this metadata can differ between operating systems. Usually the manifest is generated by the operating system based off of header data from the file itself, and details about the file that the operating system can deduce, such as file size, origin, location, file type, etc. In Windows you can view this info by right clicking/opening the context menu on any file and selecting “Properties”, on macOS by opening the context menu and selecting “Get Info”, and on other OSes such as linux/freeBSD it will be something similar.
There are other usages for “manifest” depending on the context, for example a manifest.xml would be something a developer would include with an android app that has configuration settings and properties for the app.