\documentclass{article}
\usepackage[utf8]{inputenc}
\usepackage{lipsum}
\newcommand{\mytext}{Here is my own dummy text command to fill-up space over the document. The text has no sense and you can continue reading, nevertheless, you will not find useful information. Please stop reading.}
\title{Font styles and decalrations}
\author{Gerardo Marx Chávez-Campos}
\date{February 2021}

\begin{document}

\maketitle

\section{Font Changing Text-Block Commands}

\textrm{Roman text}\newline
\textsf{San Serif text}\newline
\textbf{Bold}\\
\texttt{Type Writter}\\
\textit{Italic text}\\
\textup{Up and Right}\\
\textsc{Small Capitals}\\
\underline{Underlined text}\\

\section{Font changing declarations}
\subsection{Environments}
\begin{sffamily}
The text here will change.
\end{sffamily}
\subsection{Localised declarations}
{\sffamily\slshape Localised declarations: \lipsum[1]}. Here declarations do not affect.

\itshape Declarations  that later are reset: \lipsum[1] \normalfont. Normal text here again.

Mixing text-block and modal commands: \textsc{\rmfamily \lipsum[3]}

\textbf{\textit{Nested declarations: \lipsum[6]}}

\section{Font Size}

\begin{tiny}
Some tiny text.
\end{tiny}

\scriptsize{\lipsum[5]}

\footnotesize{\lipsum[10]}

\small{\lipsum[99]}

\normalfont{\lipsum[12]}

\large{\mytext}

\Large{\mytext}

\LARGE{\mytext}

\huge{\mytext}


\Huge{\mytext}
\end{document}