Depending on the web server the browser will usually open index.html, default.html, et cetera when you go to a page that ends with a ‘/’.
That is in an environment that is NOT using a rewriter. Once you get into rewritten URLs, you can make no assumptions about the naming conventions on the site and the pages that power them.
And I would guess that rewritten URLs are probably the default at this point on sites that you are looking at.
On your own Linux server the default would be as you described, an index.html file in the named directory. But you can enable rewriting and change the beavior.