Document templates
Although illustrative material, perhaps these document templates will be useful. Additional template resources are listed at https://tug.org/interest.html#latextemplates.

beamer template
The beamer class creates presentation slides. It has a vast array of features, but here is a basic template:

\documentclass{beamer}

\title{Beamer Class template}
\author{Alex Author}
\date{July 31, 2020}

\begin{document}

\maketitle

% without [fragile], any {verbatim} code gets mysterious errors.
\begin{frame}[fragile]
 \frametitle{First Slide}

\begin{verbatim}
  This is \verbatim!
\end{verbatim}

\end{frame}

\end{document}

The Beamer package on CTAN: https://ctan.org/pkg/beamer.

article template
A simple template for an article.

\documentclass{article}
\title{Article Class Template}
\author{Alex Author}

\begin{document}
\maketitle

\section{First section}
Some text.

\subsection{First section, first subsection}
Additional text.

\section{Second section}
Some more text.

\end{document}

book template
This is a straightforward template for a book. See Larger book template, for a more elaborate one.

\documentclass{book}
\title{Book Class Template}
\author{Alex Author}

\begin{document}
\maketitle

\chapter{First}
Some text.

\chapter{Second}
Some other text.

\section{A subtopic}
The end.

\end{document}

Larger book template
This is a somewhat elaborate template for a book. See book template, for a simpler one.
This template uses \frontmatter, \mainmatter, and \backmatter to control the typography of the three main areas of a book (see \frontmatter & \mainmatter & \backmatter). The book has a bibliography and an index.
Also notable is that it uses \include and \includeonly (see Splitting the input). While you are working on a chapter you can comment out all the other chapter entries from the argument to \includeonly. That will speed up compilation without losing any information such as cross references. (Material that does not need to come on a new page is brought in with \input instead of \include. You don’t get the cross reference benefit with \input.)

\documentclass[titlepage]{book}
\usepackage{makeidx}\makeindex

\title{Book Class Template}
\author{Alex Author}

\includeonly{%
% frontcover,
  preface,
  chap1,
% appenA,
  }

\begin{document}
\frontmatter
\include{frontcover}
  % maybe comment out while drafting:
\maketitle \input{dedication} \input{copyright}
\tableofcontents
\include{preface}

\mainmatter
\include{chap1}
...
\appendix
\include{appenA}
...

\backmatter
\bibliographystyle{apalike}
\addcontentsline{toc}{chapter}{Bibliography}
\bibliography
\addcontentsline{toc}{chapter}{Index}
\printindex

\include{backcover}
\end{document}