Galleries created by WP-Juicebox do not use static 'config.xml' files.
All XML data is generated dynamically on-the-fly at the time the gallery is displayed/viewed (so there are no XML files that you can edit manually).
WP-Juicebox generates XML data dynamically to prevent galleries from having to be rebuilt (via a separate user-driven action) when images are added to pages/posts or when image properties (such as titles) are changed.
This behavior is also in keeping with WordPress itself where content is generated dynamically.

As an alternative to using WP-Juicebox, you could create a gallery with JuiceboxBuilder-Pro (so that you have a regular gallery with a 'config.xml' file that you can edit inb a plain text editor) and then embed it into ypour WordPress page/post manually using the baseUrl method of embedding documented here.

(1) Enter your gallery's baseUrl embedding code into a 'Formatting -> Custom HTML' Gutenberg block.

(2) Upload your complete gallery folder to your root directory and use a leading slash in your embedding code paths to denote your root directory. This should work with any permalink structure. For example:

<!--START JUICEBOX EMBED-->
<script src="/gallery_folder/jbcore/juicebox.js"></script>
<script>
    new juicebox({
        baseUrl: "/gallery_folder/",
        containerId: "juicebox-container",
        galleryWidth: "100%",
        galleryHeight: "600",
        backgroundColor: "#222222"
    });
</script>
<div id="juicebox-container"></div>
<!--END JUICEBOX EMBED-->

I hope this helps.