Braces inside or outside?

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\begin{document}
\verb|\flushright| is really starting an environment and would be better marked as such in \LaTeX.

\verb|\begin{flushright}lorem\end{flushright}| \begin{flushright}lorem\end{flushright}

\verb|{\textbf lorem}| is not the same as \verb|\textbf{lorem}|.
The former will make only the \verb|l| bold, while the latter will make \verb|lorem| bold.
\verb|\bfseries| is a \emph{switch} and does not take an argument.
Everything following will be bold until the switch is changed or the local \TeX{} group ends.
Hence, limiting its scope requires a local group, \verb|{\bfseries lorem}|.

\begin{tabular}{ccc}
  \verb|\textbf{lorem}| & \verb|{\textbf lorem}| & \verb|{\bfseries lorem}| \\
  \textbf{lorem} & {\textbf lorem} & {\bfseries lorem} \\
\end{tabular}

\verb|\scriptsize| is another switch: everything following will be in this size, until another size is activated or the local \TeX{} group ends.
{\scriptsize Hence, brackets are needed to limit the scope, \verb|{\scriptsize lorem}|.
However, to be fully effective, the scope of size-changing macros must include a paragraph break at the end.}
In some contexts, this doesn't matter (and can even be exploited), but generally, ending a size mid-paragraph is not what you want.

\scriptsize \verb|\scriptsize| is another switch: everything following will be in this size, until another size is activated or the local \TeX{} group ends.
{\normalsize Hence, brackets are needed to limit the scope, \verb|{\scriptsize lorem}|.
However, to be fully effective, the scope of size-changing macros must include a paragraph break at the end.}
In some contexts, this doesn't matter (and can even be exploited), but generally, ending a size mid-paragraph is not what you want.

\normalsize
\verb|\tt| is obsolete and ought not be used in \LaTeXe{} at all.
Use \verb|\ttfamily| as a switch, \verb|{\ttfamily lorem}|, or \verb|\texttt| with an argument, \verb|\texttt{lorem}|.

If you stick to \LaTeX{} font selection commands, then all those which take arguments have names beginning with \verb|\text| (with the exception of \verb|\emph|, as explained below), while switches do not.
Switches instead have standard endings which indicate the kind of thing they switch: \verb|size| for size, \verb|family| for family, \verb|shape| for shape and \verb|series| for width and weight.

\newcommand*\cs[1]{\texttt{\textbackslash#1}}
\begin{center}
    \begin{tabular}{*{7}{l}}
      \toprule
      Switch & Switch & Text & Switch & Text & Switch & Text \\
      \midrule
      \cs{tiny} & \cs{upshape} & \cs{textup} & \cs{rmfamily} & \cs{textrm} & \cs{mdseries} & \cs{textmd} \\
      \cs{scriptsize} & \cs{itshape} & \cs{textit} & \cs{sffamily} & \cs{textsf} & \cs{bfseries} & \cs{textbf} \\
      \cs{footnotesize} & \cs{slshape} & \cs{textsl} & \cs{ttfamily} & \cs{texttt} \\
      \cs{small} & \cs{scshape} & \cs{textsc}  \\
      \cs{large} \\
      \cs{Large} \\
      \cs{LARGE} \\
      \cs{Huge} \\
      \cs{normalsize} & \cs{normalfont} & \cs{textnormal} & \cs{normalfont} & \cs{textnormal} & \cs{normalfont} & \cs{textnormal} \\
      \bottomrule
    \end{tabular}
\end{center}

This system is the New Font Selection Scheme, introduced with \LaTeXe more than twenty years ago (so no longer so `new').

\verb|\emph{lorem}| is not part of this scheme because, strictly speaking, it does not specify a particular font change but is, rather, \emph{semantic} mark-up.
\verb|\emph| and the \TeX{} switch \verb|\em| specify that text should be \emph{emphasised} using whatever format is currently active for marking such text.
By default, this means that text is italicised, if currently non-italic, or made upright, if currently italic.
However, it would be perfectly legitimate to design a class in which important text was made bold rather than rendered italic/non-italic.

With the default settings, the difference between the semantic \verb|\emph| and the non-semantic \verb|\textit| can be observed by seeing what happens when such changes are nested.

\begin{verbatim}
This is not especially important.
\emph{This is very important, especially \emph{this phrase}, which deserves particular attention.}

This is not especially important.
\textit{This is very important, especially \textit{this phrase}, which deserves particular attention.}

This is not especially important.
\emph{This is very important, especially \textit{this phrase}, which deserves particular attention.}

This is not especially important.
\textit{This is very important, especially \emph{this phrase}, which deserves particular attention.}
\end{verbatim}
This is not especially important.
\emph{This is very important, especially \emph{this phrase}, which deserves particular attention.}

This is not especially important.
\textit{This is very important, especially \textit{this phrase}, which deserves particular attention.}

This is not especially important.
\emph{This is very important, especially \textit{this phrase}, which deserves particular attention.}

This is not especially important.
\textit{This is very important, especially \emph{this phrase}, which deserves particular attention.}
\end{document}

In addition to cfr's answer I think it's important to stress that declarations do not have braces, they are not "inside their braces", they just change the state (typically the font) at the current point

So taking your examples

{\scriptsize lorem}

the outer braces are not part of the command syntax, you could just as well go

\scriptsize lorem \normalsize

or

\begin{center}\scriptsize lorem\end{center}

---

\flushright{lorem}

The braces here are unconnected with the command and just form a group that would scope any declaration such as \scriptsize inside "lorem". \flushright is only intended to be used as an environment

\begin{flushright} lorem\end{flushright}

The declaration form is \raggedright.

\raggedright lipsum

---

{\textbf lorem}

the braces here are again unconnected to the \textbf command and form a group, \textbf is a macro with one argument so here, as its argument isn't braced, the argument is the next token, which is l so it is the same as

{\textbf{l}orem}

with the l being the argument to \textbf and the outer braces playing no role in this command.

---

\em and \tt are declarations like \raggedright (\tt is not defined by default in latex and is just defined in some classes with compatibility with early versions of LaTeX)