tldr; Open the titlepages folder, then open the bg-image folder, then open article.qmd. Scroll to the pdf format section of the yaml. Then open before-body.tex. That is what is giving you control of the frontmatter.
This template makes a custom title page using the information in the YAML (from the xyz.qmd for an single doc and from index.qmd for a Quarto book). Note, this is not the quarto_titlepages extension. This repo is for LaTeX users who want to DYI a titlepage with their own Pandoc template partials. The Quarto extension lives here: quarto_titlepages.
This work is uses Quarto (citation). The default document classes for Quarto are the KOMA-script scrbook and scrartcl document classes. This repo also includes a copy of the Springer svmono document class, CRC/Chapman & Hall krantz document class, and the Elsevier elsarticle document class.