Note: If you insist in staying with class extbook instead of switching to scrbook you still can use \DeclareTOCEntryStyle if you add package scrextend. But in this case, you would need to use \DeclareTOCEntryStyle also to change the indent and numwidth to configure the flat style manually. However in this case you'd still have an issue with part entries, because these are not using \numberline. See my answer to Issue with aligning part numbers and titles in ToC using tocloft for more information about the needed configuration with book or extbook and how to patch \@part to solve the \numberline problem. So with extbook you would need something like:
\documentclass[9pt]{extbook} \usepackage[svgnames]{xcolor} \usepackage[ngerman]{babel} \usepackage{mwe} \usepackage{iftex} \usepackage{libertinus} \usepackage{amsfonts} \usepackage{unicode-math}% I recommend LuaLaTeX! \usepackage{tocbasic} \renewcommand\familydefault{\sfdefault}% Use sf as default (just a guess because all entries in the screenshot seems to be sf) % Change indent of entries \DeclareTOCStyleEntries[indent=0pt,numwidth=25pt]{tocline}{part,chapter,section,subsection} % Change fonts of ToC entries \DeclareTOCStyleEntry[entryformat=\textbf]{tocline}{section} \DeclareTOCStyleEntry[entryformat=\large\textbf]{tocline}{chapter} \DeclareTOCStyleEntry[entryformat=\Large\textbf]{tocline}{part} % Make it work also for part entries \usepackage{xpatch} \makeatletter \xpatchcmd{\@part}{% \addcontentsline{toc}{part}{\thepart\hspace{1em}#1}% }{% \addcontentsline{toc}{part}{\numberline{\thepart}#1}% }{\typeout{\string\@part\space patch successfull}}{\PaTchFailue} \DeclareTOCStyleEntry[pagenumberbox=\@gobble]{tocline}{part} \makeatother \usepackage[colorlinks, linkcolor=DarkBlue]{hyperref}% Load it as late a possible! \begin{document} \frontmatter \tableofcontents \mainmatter \part{Grundlagen: Matrizen und lineare Gleichungssysteme} \chapter{Matrizen} \section{Matrizen (Grundlagen)} \subsection{Reelle Matrizen} \subsection{Matrizen auf beliebigen Körpern} \subsection{Die Menge $\mathbb{K}^{m × n}$} \section{Operationen auf Matrizen} \subsection{Addition und Substraktion von Matrizen} \chapter{Lineare Gleichungssysteme} \end{document}
to change the ToC entries.
An additional package like titlesec would be needed to change the chapter headings (but this would IMHO be another question). So it is usually much easier to use a configurable class like scrbook.
Note: I've not changed the page header, because this would be a completely different question. If you need such changes too, please have a look into the scrlayer-scrpage chapters in the KOMA-Script documentation and the Wiki or ask a new question.
Note: extbook is derived from a very old version of standard class book. No one of the changes in book from the last 25 years have been inherited to extbook.
