Git has an handy feature that you can get a system-wide .gitignore file. This is particularly helpful to keep your project .gitignore files lean. However, Git was originally built for Linux and sort of begrudgingly supports Windows, which sometimes leads to oddities (like this):
Where do you put your global .gitignore file on Windows to automatically have it loaded?
In the end, I found the answer on an issue for a random project (Broot Issue #212):
it is stated in the [Git] documentation that
$XDG_CONFIG_HOMEis not set) is the default value of core.excludesFile.
So, translated to Windows-ese, the answer is:
ignore is the filename. The contents of the file are the same as any other .gitignore file, except they will be applied to all your projects.
Hope this is helpful to someone else!