The file format for (single) exported equations is PDF. PDFs created by the LaTeX Equation Editor store their LaTeX source code inside the PDF in a format readable by both Equation Services and the LaTeX Equation Editor.
Libraries are saved as packages - directories which look like single files in the Finder. If you control-click on a library package and select "Show package contents" you can browse the individual equations in the library. Each equation in the library is saved as a single PDF file in the package.