What you are describing is the Splash Page.
The Splash Page is an image placeholder for the gallery which, when clicked, will expand the gallery to fill the browser viewport (to give the gallery as much room as possible).
By default, the Splash Page will be displayed on mobile devices when a gallery is embedded in a web page alongside other content (where the gallery might be too small to use).
Check out the Gallery Tour where there is a short section about the Splash Page.

Please check out the Splash Page configuration options here.

By default, Juicebox uses the first image in the gallery as the Splash Page image. You can change this if you like via the splashImageUrl option (in JuiceboxBuilder-Pro's 'Customize -> Splash Page' section).
You could also choose to not use the Splash Page at all by setting showSplashPage="NEVER".

Whether you use the Splash Page or not, on a mobile device, the gallery will be displayed in Small Screen Mode (check out the Gallery Tour link above for details) where thumbnails and main images are displayed on separate pages (to give the main images more room).
If you'd rather have the gallery displayed in Large Screen Mode (where thumbnails are displayed alongside the main images) on all devices and in all browsers, then you can set screenMode="LARGE" (in JuiceboxBuilder-Pro's 'Customize -> General' section).

Your gallery currently sets maxCaptionHeight="70".
Try increasing the maxCaptionHeight (at least to the default value of 120) to give your captions more room. You'll find maxCaptionHeight in JuiceboxBuilder-Pro's 'Customize -> Caption' section.

Also, your gallery's first caption is:

<caption><![CDATA[</font><font color="#CCCCCC">Projet: Pecquencourt - Production: <font color="#0066FF"><a href="https://mw-architecture.fr/">MW Architecture </a><font color="#CCCCCC"> - Date: Novembre 2022  </font><br><font color="#0066FF"><a href="http://www.vincefx.com">www.vincefx.com </a><font color="#CCCCCC"> vincefx@free.fr tel:06.98.72.06.93]]></caption>

For a valid HTML 5 page, I'd recommend changing <font> tags for equivalent <style> tags as noted here (a little bit of work would be required but the resulting code would be HTML 5 compliant). Also, there's no need to start the caption with a closing </font> tag and you're missing a few </font> tags throughout your caption text.

Your gallery's embedding page actually has two galleries embedded into it (two lots of embedding code with with same 'juicebox-container' id).
I expect that you're just trying to embed one gallery so you should be able to remove one set of embedding code.
If you are trying to embed multiple galleries into a single web page, then you only need to load the 'juicebox.js' JavaScript file once per page (not once per gallery) and each gallery should be loaded into a <div> with a unique id.

I hope my notes above point you in the right direction.

At the moment, your gallery looks good in both portrait and landscape orientations in Mobile Safari on my iPhone and in Mobile Chrome on an Android device.
If things look a little odd, then try clearing your browser's cache before reloading your gallery's web page to see if this helps to resolve the problem.

It's usually a good idea to set screenMode="AUTO" (the default value) and allow Juicebox to determine whether to display the gallery in Large Screen Mode or Small Screen Mode (depending on the device being used to view the gallery).

In Small Screen Mode (whether you are in Fullscreen Mode or not), you can display the Thumbnail Button on the gallery's Button Bar (to allow the visitor to view the thumbnail pages) by setting showSmallThumbsButton="TRUE" (in JuiceboxBuilder-Pro's 'Customize -> Thumbnails' section).
You will need to display the Button Bar in your gallery, though, in order for the Thumbnail Button to be displayed (and your gallery currently hides the Button Bar by setting buttonBarPosition="NONE").

Your first image's caption is currently the following:

</font><font color="#CCCCCC">Projet: Pecquencourt - Production: <font color="#0066FF"><a href="https://mw-architecture.fr/">MW Architecture </a><font color="#CCCCCC"> - Date: Novembre 2022  </font><br><font color="#0066FF"><a href="http://www.vincefx.com">www.vincefx.com </a><font color="#CCCCCC"> vincefx@free.fr tel:06.98.72.06.93

You could change it to this to be HTML 5 compliant:

<p><span style="color: #cccccc;">Projet: Pecquencourt - Production: </span><a href="https://mw-architecture.fr/" style="color: #0066ff;">MW Architecture</a><span style="color: #cccccc;"> - Date: Novembre 2022 </span><br><a href="http://www.vincefx.com" style="color:#0066ff;">www.vincefx.com</a><spa style="color: #cccccc"> vincefx@free.fr tel:06.98.72.06.93</span></p>

This is HTML 5 compliant but it might not make a visible difference. Most browsers will likely still render the <font> tag OK and may be quite tolerant towards things like missing end tags.
If you don't see a visible difference, then you might not want to bother changing all your captions just now (as it is quite a large task) but it would be a good idea going forward (especially as your web page uses the HTML 5 Doctype Declaration).

Yes. The embedding page is your 'ImagesArchi.html' file.

I never actually saw two galleries on the page but there were two sets of embedding code (both embedding the same gallery into the same container).
Your web page only has one set of embedding code now, so that's good.

Your gallery's Button Bar displays OK in my own desktop browsers and on my Android phone.
If you don't see the Button Bar yourself, then try clearing your browser cache before reloading your gallery's web page to ensure that the browser is loading the gallery's 'confg.xml' file fresh from your web server (with the new setting for buttonBarPosition) instead of using an older, cached version.

Incidentally, on mobile in portrait orientation, there's not much room for the Button Bar (due to the length of the Gallery Title) so the Button Bar spills over onto several lines. Try shortening the Gallery Title.
Also, in Small Screen Mode (on mobile devices), please note that the Button Bar is displayed only on main image pages and not on thumbnail pages.