tabularray: How to control the vertical alignment of the cells contents?

Question

For the following, I would like to understand

1- what rows={b}, columns={b}, or cells={b} should actually do

2- why none of them makes the contents of the rows bottom-aligned as instructed by the option b.

\documentclass{article}
\usepackage[paperheight=75mm]{geometry}
\usepackage{xcolor}
\usepackage{tabularray}
\UseTblrLibrary{booktabs}
\begin{document}
\begin{tblr}[long, caption = {The Caption II}]
    {
        rows = {5.0ex, b},
        cells = {b},
        columns = {b},
        hlines = {white,1.0pt},
        rulesep = 0pt,
        width = 0.5\linewidth,
        colspec = {XX}, 
        row{odd} = {blue!15}, 
        row{even} = {gray!15},
        rowhead = 1,
        row{1} = {red!20},
    }
    \toprule[1.5pt] 
    \hline[white,1.0pt]
    One & Two  \\
    \midrule
    \hline[white,1.0pt]
    1  &   2  \\
    3  &   4  \\
    5  &   6  \\
    7  &   8  \\
    9  &  10  \\
    \bottomrule[1.5pt]
\end{tblr}

\end{document}

Answer 1

See my answer in the question p,m and b columns in tables. What you really want is f alignment.

\documentclass{article}
\usepackage[paperheight=75mm]{geometry}
\usepackage{xcolor}
\usepackage{tabularray}
\UseTblrLibrary{booktabs}
\begin{document}
\begin{tblr}[long, caption = {The Caption II}]
    {
        rows = {5.0ex, f},
        hlines = {white,1.0pt},
        rulesep = 0pt,
        width = 0.5\linewidth,
        colspec = {XX}, 
        row{odd} = {blue!15}, 
        row{even} = {gray!15},
        rowhead = 1,
        row{1} = {red!20},
    }
    \toprule[1.5pt] 
    \hline[white,1.0pt]
    One & Two  \\
    \midrule
    \hline[white,1.0pt]
    1  &   2  \\
    3  &   4  \\
    5  &   6  \\
    7  &   8  \\
    9  &  10  \\
    \bottomrule[1.5pt]
\end{tblr}

\end{document}