$ pandoc in.md  -o out.pdf --pdf-engine=lualatex  -V mainfont="Palatino" -H header.tex

• in.md is the source file
• -o ... is the output file (out.pdf here)
• --pdf-engine=lualatex is the latex plugin
• -H is the latex header file (header.tex here) that contains latex commands that are prepended to your input file (in.md)

\usepackage{fancyhdr}
\usepackage{lastpage}

\newcommand\hffont{\fontsize{08pt}{08pt}\selectfont}

\pagestyle{fancy}
\fancyhf{}
\lhead{\hffont{Unit 3.7: Assessment for feedback}}
\rhead{\hffont{SBI4U}}
\lfoot{\hffont{Jeffrey Shevlen 99419720}}
\fancyfoot[R]{\hffont{\thepage\ of \pageref{LastPage}}}
\renewcommand{\headrulewidth}{0.3pt}
\renewcommand{\footrulewidth}{0.3pt}

• \usepackage{...} loads latex relevent packages
• newcommand is a user/me generated thing called hffort where I specify the header and fooder size
• no idea what pagestyle{fancy}, fancyhf{} do
• left head, right foot are \lhead{} and \rfoot{}
• \fancyfoot[R]{...} is like \rfoot, the [R] for right
• \renewcommand is a bit of a mystery to me
• \headrulewidth{} sets header line width

You don't need to add anything in Obsidian anyway. The -H ... pandoc arg is enough.

You may see something like this on the web.

---
output:
  pdf_document:
    includes:
      in-header: header.tex
---

Didn't work for me in this context.

• \newcommand comes by way of https://latex-tutorial.com/changing-font-size/
• the fancyheader manual http://distrib-coffee.ipsl.jussieu.fr/pub/mirrors/ctan/macros/latex/contrib/fancyhdr/fancyhdr.pdf
• markdown header reference https://stackoverflow.com/a/40992929