\documentclass[a4paper]{scrartcl}
\usepackage{amssymb, amsmath} % needed for math
\usepackage[utf8]{inputenc} % this is needed for umlauts
\usepackage[ngerman]{babel} % this is needed for umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
\usepackage[margin=2.5cm]{geometry} %layout

\usepackage{siunitx}
\usepackage{booktabs}

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\begin{document}

    \newcolumntype{+}{>{\global\let\currentrowstyle\relax}}
    \newcolumntype{^}{>{\currentrowstyle}}
    \newcommand{\rowstyle}[1]{\gdef\currentrowstyle{#1}%
    #1\ignorespaces
    }

    \begin{table}[ht]
        \centering
        \begin{tabular}{+l|^r^r^r}
        \toprule
        \rowstyle{\bfseries}%
        Country / Property & Population       & Area     & HDI \\\midrule
        France             & $66 \cdot 10^6$  & $\SI{668763}{\km\squared}$  & 0.89 \\
        Germany            & $81 \cdot 10^6$  & $\SI{357167}{\km\squared}$  & 0.92 \\
        United States      & $317 \cdot 10^6$ & $\SI{9629091}{\km\squared}$ & 0.94 \\\bottomrule
        \end{tabular}
        \caption{Information about countries}
        \label{table:countries}
    \end{table}
\end{document}
