documentclass[11pt]{book}
usepackage{lmodern}
usepackage[T1]{fontenc}
usepackage{xcolor}

usepackage{amsmath}
usepackage{amsbsy}
usepackage{amssymb}
usepackage{bm}
usepackage{fixmath}

usepackage{titlesec}
    titleformat{subsection}[runin]{normalfontbfseries}{hspace{10mm}arabic{subsection}.}{1mm}{}{}
%   titleformat*{chapter}{bfseriesboldmath}%<<==BERNARD This causes an error
    titleformat*{section}{bfseriesboldmath}%<<==BERNARD This shrinks titles
    usepackage[detect-all]{siunitx}%<<<<<<<=======BERNARD
usepackage[dotinlabels]{titletoc}
    titlecontents{chapter}[6pc]{addvspace{1pc}bfseriesfilright}{contentslabel[chaptername thecontentslabel]{6pc}}{}{hfillcontentspage}[addvspace{2pt}]%
    newcommandxquad{hspace*{0.4em plus .2em minus .2em}}%
    titlecontents*{p-section}[0pt]{filrightsmall}{}{}{, thecontentspage}[xquadtextbulletxquad][.]%
    newcommand{ChapterToc}{startcontents[chapters]printcontents[chapters]{p-}{1}{}vspace{10mm}}%
        setcounter{tocdepth}{1}% <- only parts, chapters and sections in tocs  hspace{10mm}

usepackage{tcolorbox}%
    tcbuselibrary{%Begin load libraries for tcolorbox
        xparse,%
            }%End load libraries for tcolorbox
usepackage{makeidx} %The imakeindex does not work
        makeindex

    usepackage[hyperfootnotes=false]{hyperref}
%=============== Begin the Heiko Oberdiek ``quick and dirty hack'' which loads showidx
makeatletter
ifHy@hyperindex
  defHyInd@ParenLeft{(}%
% Hook in HyInd@@wrindex
  letHyInd@showidx@empty
% Hook in HyInd@@wrindex for package showidx
  defHyInd@showidx#1{%
    @showidx{#1}%
    if@nobreak
      ifvmode
        nobrak
      fi
    fi
  }%
% Load package showidx
  letsiOrg@makeindexmakeindex
  letsiOrg@@index@index
  letsiOrg@@wrindex@wrindex
  letsiOrg@indexindex
  RequirePackage{showidx}
  letmakeindexsiOrg@makeindex
  let@indexsiOrg@@index
  let@wrindexsiOrg@@wrindex
  letindexsiOrg@index
% rest of hyperref part
  @ifpackageloaded{multind}{%
    letHyInd@org@wrindex@wrindex
    def@wrindex#1#2{HyInd@@wrindex{#1}#2||\}%
    defHyInd@@wrindex#1#2|#3|#4\{%
      ifx\#3\%
        HyInd@org@wrindex{#1}{#2|hyperpage}%
      else
        defHy@temp@A{#3}%
        ifxHy@temp@AHyInd@ParenLeft
          HyInd@org@wrindex{#1}{#2|#3hyperpage}%
        else
          HyInd@org@wrindex{#1}{#2|#3}%
        fi
      fi
    }%
  }{%
    def@wrindex#1{@@wrindex#1||\}
    def@@wrindex#1|#2|#3\{%
      ifx\#2\%
        protected@write@indexfile{}{%
          stringindexentry{#1|hyperpage}{thepage}%
        }%
      else
        defHy@temp@A{#2}%
        ifxHy@temp@AHyInd@ParenLeft
          protected@write@indexfile{}{%
             stringindexentry{#1|#2hyperpage}{thepage}%
          }%
        else
          protected@write@indexfile{}{%
            stringindexentry{#1|#2}{thepage}%
          }%
        fi
      fi
      endgroup
      HyInd@showidx{#1}%
      @esphack
    }%
  }%
fi
makeatother
%=============== End the Heiko Oberdiek ``quick and dirty hack'' which loads showidx
    hypersetup{%begin colors
                   colorlinks,
                   linkcolor={red!50!black},
                }%end colors

usepackage{cleveref}% should be the last package
    Crefname{subsection}{Subsection}{Subsections}
    newcommand{titleref}{nameref}%displays TITLE of refered item
    setcounter{tocdepth}{1}% <- only parts, chapters and sections in tocs

%========================== Begin math in Showindex
letoldopenparen(
letoldcloseparen)
def({protectoldopenparen}
def){protectoldcloseparen}
%========================== End math in Showindex
%========================== Begin Andrew code for ref to indexed words as edited Oct 27 at 17:24
    newcommand{INDEX}[2][]{phantomsection%
    ifrelaxdetokenize{#1}relaxindex{#2}label{ind:#2}%
   elseindex{#2}label{ind:#1}fi%
    }%
%========================== End Andrew code for ref to indexed words as edited Oct 27 at 17:24
%===Begin DEX original
    newcommand{DEX}[2][]{%
   ifrelaxdetokenize{#1}relaxhyperref[ind:#2]{#2}%
   % (Cpageref{ind:#2})%
   elsehyperref[ind:#1]{#2}%
   %(Cpageref{ind:#1})%
   fi%
}%
%%===End DEX original
%============== Begin To make Cpageref work with INDEX
newcommand{PGX}[2][]{%
   ifrelaxdetokenize{#1}%
   relax%
%   hyperref[ind:#2]{#2}%
    Cpageref{ind:#2}%
   else%
%   hyperref[ind:#1]{#2}%
   Cpageref{ind:#1}%
   fi%
}%
%============== End To make Cpageref work with INDEX.
begin{document}
         tableofcontents
chapter{Chapter Title emboldens %
0 and $infty$ but not $0$ nor $x_{0}$ nor $MathText$%
}%
label{1-0-0}
    ChapterToc
Some text
 with an indexed textbf{word}INDEX[]{word} and indexed math $boldsymbol{x_{0}}$INDEX[x0]{$x_{0}$}:

section{Section Title emboldens %
 0 and $infty$ as well as $0$ and $x_{0}$ and $MathText$%
 }%
 label{1-1-0}
 A bit more text and finally:


subsection{%
Subsection Title emboldens %
0 and $infty$ %
but not $0$ nor $x_{0}$ nor $MathText$. %
However %
$boldsymbol{MathText}$, %
$boldsymbol{x_{0}}$ and %
$bm{0}$ all work.%
}%
label{1-2-1}
Plus just a tiny little bit of text%
 with a referenced DEX[word]{word} %
which was indexed in Cref{1-0-0} nameref{1-0-0} on Cpageref{1-0-0}

    phantomsection%otherwise, clicking on index in toc gets to GNU
    addcontentsline{toc}{chapter}{Index}
    printindex
end{document}