\documentclass[
german,
fontsize=11pt,
paper=a6,
landscape,
]{scrartcl}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\newcounter{cntA}\setcounter{cntA}{0}
\newcounter{cntB}\setcounter{cntB}{0}
\newcounter{cntC}\setcounter{cntC}{0}
\newcommand{\usecnt}{%
    \edef\letter{\Alph{subsection}}%
    \refstepcounter{cnt\letter}%
    \arabic{cnt\letter}%
}

\newcommand{\addpageauto}[2]{%
    \begin{flushleft}
        \fbox{\usecnt}~
        \begin{minipage}[t]{0.9\textwidth}
            {#1}%
    \end{minipage}\end{flushleft}\newpage%
    \noindent{%
        #2}%
    \newpage
}

\begin{document}

\section{The first field/topic, Walls}

\subsection{The first subsection is from a specific literature source, John Smith}

\addpageauto{Walls are\ldots}{}

\subsection{Content in the second subsection(s) is always from a certain institute}

\addpageauto{Wall can be\ldots}{incredibly boring to observe. They just stand still!}

\subsection{The third subsection is for another author, namely Jane Doe}

\addpageauto{Walls are great, but\ldots}{need some proper building first.}

\addpageauto{The 2nd flashcard by Jane Doe about walls}{No idea.}

\addpageauto{Another flashcard to memorize}{How much can someone write about walls, really??}

\section{The second field/topic, Trees}

\subsection{Again the first subsection with content by John Smith}

\addpageauto{Yes}{No}

\subsection{The second subsection is again from content by that certain institute}

\addpageauto{Leafs}{Green}

\subsection{The third subsection for Jane Doe's comments on the matter}

\addpageauto{Trees are not}{my thing. Too woody.}

\end{document}