Dominik,

Are you referring to template / layout overrides for the product and category pages?

If so, depending on what layout plugin you are using under the Apps section, (I believe it comes loaded with Bootstrap 4, but it should be Bootstrap 5. Either way, the easist way to create template layout overrides for those files would be:
In your Joomla template folder (website directory/templates):

• Create a new folder structure inside <template name>/html/
• com_j2store/templates

So the final structure should be:
root dir/templates/<template folder>/html/com_j2store/templates

Next step is inside the plugins folder look for:
root dir/plugins/j2store/app_bootstrap5 (or whatever plugin you are using) and then grab the bootstrap5 and tag_bootstrap5 folders from:
root dir/plugins/j2store/app_bootstrap5/app_bootstrap5/tmpl 

and copy/paste them into the folder you created in the first step above.

The files that start with default_ control the layout of the category pages and the files the start with view_ control the layout of the product detail pages.

Hope this helps!

Adam