directory browsing/listing as I have seen it is a function of the webserver - what you want in your 1st post is what I usually see happen when running jekyll serve - if you pull up a directory only (no index.html file) I usually see a list of files - but I have all my files output at index.html inside a folder with the title - so some-file.html would not exist - it would be some-file/index.html.
If in the browser I put /some-directory/ I would get a file list (haven’t done this in a long time so could be wrong now) as there would be no index file there and the built in webserver for jekyll I think does have enable directory listing turned on. But that is just locally. My understanding is that this is a really bad practice to have this enabled for public websites as it would show anything and everything which could show all kinds of files with secret keys and what not.
So if you want this locally, I think it is possible and may have to do with the permalink structure if it is not working for you.
If you want this on GH then I don’t think it is going to happen without you somehow creating it which I can’t see an easy way to do.