Inconsistency between Xenops external command input and latex engine

[deshearth]

In tex file generated by Xenops (located at /var/folder/... and it can be found by click copy failing command), it uses a package called "fontspec". But the compiling engine is latex. So if I run failing command in terminal, the error is that lualatex or xelatex is required to use the package. I wonder how this tex file is generated and is there any way to customize the preamble. I know that variable org-format-latex-header can be set. But in the tex file generated by Xenops, there are a few other packages, such as "fontspec".