You must use the "home" page file name that your host sets, you need to use
this default file in every folder: eg it could be "index.html" (or
index.htm, home.html, default.html etc) so, for the default page in each
folder, a page called "index.html" (etc) must be placed in each folder and
sub-folder.
The host can also turn off the directory browsing (so you don't get that
file list), but the user will see a 403 Forbidden message, unless they have
browsed directly to another html file, or unless a default index.htm page
exists in those folders.
Check with your host about which file name they have set to be used.