Final solution. Thanks to JS Bibra and his use of anchor
\documentclass[tikz,border=2pt]{standalone}
\usetikzlibrary{chains, positioning, shapes.symbols}
\usepackage{xcolor}
\definecolor{darkblue}{RGB}{9,72,90}
\definecolor{lightblue}{RGB}{124,184,201}
\begin{document}
\begin{tikzpicture}[
node distance = 2pt,
start chain = going right,
start/.style = {signal,
draw=darkblue,
very thick,
fill=lightblue,
text width=2cm,
minimum height=1cm,
font=\scriptsize,
text=white,
signal pointer angle=120, on chain},
cont/.style = {start,
signal from=west}
]
\node[start,](a) {\bfseries Construction de l'image};
\node[above=0.6cm of a.west,
anchor=south west,
rectangle,
rounded corners,
draw=darkblue,
fill=darkblue,
minimum height=2cm,
minimum width=13cm,
text=white,
inner sep=1pt]
(boitetitre){};
\node [right=1em of boitetitre.west,
anchor=south west,
text=white]
(titre) {\Large{La photographie numérique}};
\node [left=1em of boitetitre.south east,
anchor=south east,
text=white]
(titre) {\Large{Troisième}};
\node [right=1em of boitetitre.west,
anchor=south west,
text=white]
(titre) {\Large{La photographie numérique}};
\node[cont] {\bfseries Métadonnées EXIF};
\node[cont](b) {\bfseries Step3};
\node[right=5.4cm of boitetitre.north east, anchor=north east,
rectangle,
draw=darkblue,
fill=darkblue,thick,
rounded corners,
minimum height=3cm,
text width=5cm,
text=white]
(competences){Identifier les étapes de la construction de l'image finale};
\end{tikzpicture}
\end{document}