Thanks. Yes, there is an index.php. It’s annotation says
This file doesn’t do anything, but loads wp-blog-header.php which does and tells WordPress to load the theme.
That seems to be what it does, and the file wp-blog-header.php is there, too. It requires a further file, wp-load.php (also present) before the statement wp(), which (I’m guessing) should start everything off.
But it doesn’t …