where do I input this?
You can input this code in a text file with a .tex extension, for example, joinpdf.tex. Then, you can compile this joinpdf.pdf, which contains the combined contents of "file1.pdf" and "file2.pdf".
Give me the code to join two PDF files using LaTeX. a non specific way would be:

\usepackage[final]{pdfpages}

\begin{document}

\includepdf[pages=-]{file1.pdf}
\includepdf[pages=-]{file2.pdf}

\end{document}