Q:

Draw box for text

i would like to replicate this box from a french physics book. I've tried mdframed, tcolorbox, fancybox packages without success.
May someone help me please?

this is waht i've tried so far:
with fancy box:
\documentclass[10pt,a4paper]{book}
\usepackage{amsmath}
\usepackage{amsfonts}
\usepackage{amsthm}
\usepackage{amssymb}
\usepackage{graphicx}
\usepackage[top=1.5cm, bottom=1.5cm, outer=6.5cm, inner=1.5cm, heightrounded, marginparwidth=4cm, marginparsep=0.5cm]{geometry}

\usepackage{fancybox}

\begin{document}
    \newcommand{\application}[2]{\boxput *(-.7 ,1){
    \colorbox{white}{\bfseries{Application n°#1}}
    }{
    \setlength {\fboxsep }{10pt}
    \fbox {\begin{minipage}{17cm}
    #2
    \end{minipage}}
    }}
\end{document}

This box will contaain text and picture, and it would be nice if it brake at the en d of a page.
As you can see it only misses a thick horizontal line

A:

I have tried getting something from where you can work from, using tcolorbox.
Of course, I haven't matched your fonts... but that shouldn't be difficult
\documentclass[10pt]{article}
\usepackage{tikz}
\usepackage{tcolorbox}

\tcbuselibrary{skins}

\newtcolorbox{application}[2][]{%
    % [#1]: Extra options for the tcolorbox
    % {#2}: Title of box
    title= {#2},
    enhanced, 
    skin=enhancedlast,
    sharp corners,
    colframe = blue,
    colback = white,
    drop fuzzy shadow,
    boxrule = 0.41pt, %Width of line    
    borderline north={1.1mm}{-1.1mm}{black}, %The two distances must be equal magnitude but opposite in sign
    attach boxed title to top left={
        xshift = -2mm,
        yshift=-0.5mm,
        yshifttext=-1mm
    },
    coltitle = black!50!blue,
    fonttitle = \Large\itshape,
    boxed title style = {
        colframe = white,
        colback = white,
        sharp corners
    },#1
}

\begin{document}

\begin{application}{My application 1}
     hey

     it's me

     you are looking for

     \vspace{8cm}
\end{application}

\end{document}

You could get more fancy, using the auto counter option with My Application \thetcbcounter, but I am not sure if that is what you wanted, and it would only complicate the solution for not much added benefit

