Chapter title under chapter number in KOMA class

Question

Using the scrbook class, how do I place the chapter title under the chapter number? Here's a MWE

\documentclass{scrbook}
\usepackage{xcolor, graphicx}
\usepackage[showframe]{geometry}

\definecolor{chaptergrey}{rgb}{0.7,0.7,0.7}

\let\raggedchapter\raggedleft
\addtokomafont{disposition}{\normalfont}
\setkomafont{chapter}{\LARGE}

\renewcommand*{\chapterformat}{%
\scalebox{5}{\color{chaptergrey}\thechapter}%
}
\begin{document}
\chapter{A chapter}
\end{document}

The corresponding output is

And what I'd like to have is something like this

Once I can do that, how do I change the vertical space between the two?

EDIT:

\documentclass[chapterprefix]{scrbook}
\usepackage{xcolor, graphicx}
\definecolor{chaptergrey}{rgb}{0.7,0.7,0.7}

\let\raggedchapter\raggedleft
\addtokomafont{disposition}{\normalfont}
\setkomafont{chapter}{\LARGE}

\renewcommand*{\chapterformat}{%
\scalebox{5}{\color{chaptergrey}\thechapter}%
}

%\makeatletter
%\renewcommand{\@chapapp}{}
%\makeatother

\usepackage{fancyhdr}
\renewcommand{\headrulewidth}{0pt}

\fancypagestyle{theorems}{
\renewcommand{\chaptermark}[1]{\markboth{##1}{}}
\fancyhead[LO, RE]{\itshape\nouppercase\leftmark}
\fancyhead[LE, RO]{\thepage}
\fancyfoot{}}

\pagestyle{theorems}
\begin{document}
\tableofcontents
\chapter{A chapter}
\newpage
a
\end{document}

Corresponding heading

Answer 1

You can use option chapterprefix if you want a chapter heading with a prefix line, that has only the formatted number:

\documentclass[chapterprefix]{scrbook}
\usepackage{xcolor, graphicx}
\usepackage[showframe]{geometry}

\definecolor{chaptergrey}{rgb}{0.7,0.7,0.7}

\let\raggedchapter\raggedleft
\addtokomafont{disposition}{\normalfont}
\setkomafont{chapter}{\LARGE}

\renewcommand*{\chapterformat}{%
\scalebox{5}{\color{chaptergrey}\thechapter}%
}
\renewcommand*{\chaptermarkformat}{\thechapter\autodot\enskip}% If you don't want the chapter name in the running head.
\begin{document}
\chapter{A chapter}
\end{document}

There are already examples for similar chapter headings like:

• https://tex.stackexchange.com/a/480247/9057
• https://tex.stackexchange.com/a/298509/9057

Please avoid using fancyhdr with KOMA-Script classes. As the warning message tells you, using fancyhdr breaks several features of KOMA-Script classes. Use scrlayer-scrpage instead. If you want running head aligned to the inner margin and page numbers aligned to the outer margins and if the chapter titles in the running head should be without chapter numbers, you can use:

\renewcommand*{\chaptermarkformat}{}% If you want running heads without
                                % chapter number.

\usepackage[automark]{scrlayer-scrpage}
\clearpairofpagestyles
\ihead*{\pagemark}% page number also on plain pages
\ohead{\headmark}