The exact set of fonts built in to any version of MuPDF is configurable (See chapter 18 Build configuration).
For PDF, we build in a basic set of fonts licensed from URW. Originally the PDF specification suggested that 14 standard fonts should be available on all readers, and that any other fonts should be embedded within files. This recommendation has since been updated to suggest that all fonts (or at least the required subset thereof) are embedded in all files. Nonetheless it is still common to find many PDF files that ask for fonts that are not embedded.
In order to cope with the widest range of scripts possible, MuPDF is supplied with (and can optionally include) a selection of the ‘Noto’ fonts from Google.
If this would take too much space on your target system, an alternative is to use the supplied DroidSansFallback font (also from Google) which is a good trade-off between size and coverage.
MuPDF does not, by default, make use of fonts present on the underlying system (for instance, on Windows, MuPDF will not look for fonts in C:/Windows/). Should you wish to implement this kind of ‘system font’ loading, however, MuPDF does provide hooks for this to be done. The fz_install_load_system_font_funcs call takes a set of function pointers that can be used for this purpose.