I am giving Underscores a try after hearing rave reviews about it. I have a few questions about underscores I hope someone can help me with.
I found the etc/template.-tags.php file, but can’t figure out how to change the output date format for posts, and add a <br> between Categories for a post.
Can anyone help me figure out how/where to override these properly?
The date format is directly controlled by the Settings > General page. You can find date format there. At least on a default Underscores theme.
As to the categories, check for example content.php. In the entry footer you can see the following, but of course with your theme slug:
$categories_list = get_the_category_list( __( ', ', 'theme-slug' ) );
The first parameter controls the separator as explained here.
So you can just change it to:
$categories_list = get_the_category_list( __( '<br />', 'theme-slug' ) );
Now you will have breaks between categories.