@janecraft
My goal was to have a subdirectory called wordpress for the wordpress files ((https://example.com/wordpress/) and a subdirectory called dev where I’d put my pages, blog, etc (https://example.com/dev/).
You can not have two subdirectories. It doesn’t work that way. Since its working in /wordpress/, put your /dev/ files back in the /wordpress/ tree, in their original relative sub-folder locations.
WP root files in /wordpress/ and within that
/wp-content/
/wp-admin/
/wp-includes/
If that gets it working, fine. If not, you can add two lines to wp-config.php that override the site location info that’s stored in the database – the equivalent of what you did to give WP its own directory. Point them to the actual locations – you can keep the modified index.php in the site root folder if you so choose
define('WP_SITEURL', 'https://example.com/wordpress');
define('WP_HOME', 'https://example.com/wordpress');