I suspect that this is new question follows from how-to-fit-a-table-in-a-one-row. As starting point for the table showed in the first image in mentioned question can be written as follows:
\documentclass[12pt,oneside]{book}
\usepackage[showframe]{geometry}
\usepackage{ragged2e}
\usepackage{booktabs, makecell, multirow, tabularx, tabulary}
\renewcommand\theadfont{\small\bfseries} % for bold in table using \small
\renewcommand\theadgape{}
\newcolumntype{L}{>{\RaggedRight}X}
\usepackage{threeparttable} %to add footnote below table
\usepackage[svgnames, table]{xcolor}
\usepackage[skip=1ex]{caption} %for table spacing to second row
\usepackage{siunitx} %for table spacing to second row
\begin{document}
\begin{table}[ht]
\sisetup{round-mode = places,
round-precision=2,
detect-weight
}
\setlength\tabcolsep{3pt}
\begin{threeparttable}
\caption{Bootstrap for paired Samples Test}
\label{tab:blabla}
\begin{tabularx}{\linewidth}
{@{} l L *{3}{S[table-format=2.2,table-space-text-post={\,\%}]}
S[table-format=1.3]
*{2}{S[table-format=2.2,table-space-text-post={\,\%}]}
@{}}
\toprule
& & & & & & \multicolumn{2}{c}{\thead{\SI{95}{\%} Confidence\\ Interval}} \\
\cmidrule{7-8}
& & {Mean} & {Bias} & {\thead{SE\tnote{a}}} & {\thead{STT\tnote{b}}}
& {\thead{Lower}}
& {\thead{Upper}} \\
\toprule
Pair 1 & Testing example dataset (BB) - Testing Example Dataset (AA)
& 12.34567\% & 12.34567\% & 12.34567\% & 1.234 & 12.34567\% & 12.34567\% \cr
\bottomrule
\end{tabularx}
\begin{tablenotes}[para, flushleft]
\item[a] SE: Standard Error
\item[b] STT: Signal, Two Tailed
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}

On similar way you can redesign your table from this question:
\documentclass[12pt,oneside]{book}
\usepackage[showframe]{geometry}
\usepackage{ragged2e}
\usepackage{booktabs, makecell, multirow, tabularx, tabulary}
\renewcommand\theadfont{\small\bfseries} % for bold in table using \small
\renewcommand\theadgape{}
\newcolumntype{L}{>{\RaggedRight}X}
\usepackage{threeparttable} %to add footnote below table
\usepackage[svgnames, table]{xcolor}
\usepackage[skip=1ex]{caption} %for table spacing to second row
\usepackage{siunitx} %for table spacing to second row
\begin{document}
\begin{table}[ht]
\sisetup{round-mode = places,
round-precision=2,
detect-weight
}
\begin{threeparttable}
\caption{Bootstrap for paired Samples Test}
\label{tab:blabla}
\begin{tabularx}{\linewidth}
{@{} l L *{2}{S[table-format=2.2,table-space-text-post={\,\%}]}
S[table-format=1.2]
S[table-format=1.0]
S[table-format=1.3]
@{}}
\toprule
& & \multicolumn{2}{c}{\thead{\SI{95}{\%} CID\tnote{a}}}
& & & \\
\cmidrule{3-4}
& & {\thead{Lower}}
& {\thead{Upper}} & {\thead{t}} & {\thead{df}} & {\thead{STT\tnote{b}}} \\
\toprule
Pair 1 & Testing example dataset (BB) - Testing Example Dataset (AA)
& 12.34567\%
& 12.34567\% & 1.23 & 4 & 0.001 \\
\bottomrule
\end{tabularx}
\begin{tablenotes}[para, flushleft]
\item[a] CID: Confidence Interval of the Difference
\item[b] STT: Signal, Two Tailed
\end{tablenotes}
\end{threeparttable}
\end{table}
\end{document}

\makecellor\theadfrom themakecellpackage. Various possible vertical alignments of such a two lined header using the before mentioned commands, can also be found in the answers to the following question: https://tex.stackexchange.com/q/510888/134144 – leandriis Nov 03 '19 at 14:17vertically centre of the header for "t" and "df". and also making theSig. (2-tailed)into 2 line. – newlatex Nov 03 '19 at 14:17