Inner Blocks are Gutenberg Blocks that allow inserting additional blocks within your own block. To understand it even better, imagine a shortcode that wraps the content. This content can then be shown or hidden. You can achieve that inside Gutenberg with Inner Blocks.
WooCommerce has a postcode validator built in which is used to find appropriate shipping methods on the checkout. In case you are going to extend some shipping methods or build something else with postcodes, you will learn here how you can use that for your own solution.
When developing solutions such as themes, plugins or a custom solution, the database tends to get bigger and also with the defaults already saved. When adding new and complex features, to test them, databases should be empty.