I'm currently trying to loop over a subfolder in Jekyll but I don't know whether this is possible.
My folder structure looks like this:
_includes
_layouts
_pages
folder_1
folder_2
index.html
{% for page in pages.folder_1 %}
//xyz
{% endfor %}
Avoid naming main folder with an underscore unless you are using the _posts folder.
Then filter posts or pages checking their path:
{% assign folder1 = site.pages | where_exp: "item" , "item.path contains 'folder1'"%}
{% for item in folder1 %}
{{item.title}}
{% endfor %}