\KOMAoption{chapterprefix}{true}
% centering the chapter head
\let\raggedchapter\centering

\setkomafont{chapterprefix}{\normalsize\mdseries}

% This block has the two variables \chapapp and \thechapter. But where
% can I find a variable for the title of the chapter (eg. \chaptertile) to apply some styling?
\renewcommand*{\chapterformat}{%
  \kollektif{}
  \fontsize{18}{18}
  \MakeUppercase{\chapapp}~\thechapter\\[-0.6\baselineskip]
  \noindent\makebox[\textwidth]{%
    \rule{0.9\textwidth}{.5pt}%
  }%
}
\addtokomafont{chapter}{\normalfont\fontsize{26}{26}\norwester{}\MakeUppercase}

\RedeclareSectionCommand[beforeskip=0pt,afterskip=8\baselineskip]{chapter}