Are database tables/content (media) removed after sandbox expiration?

Yes, the tables and all files are removed after sandbox expiration if you defined this option in Demo Builder settings.

Some third party themes or a plugins may create custom tables then the plugin or the theme has to make sure that these tables are deleted when the sub-site is removed. We use a standard method with wpmu_delete_blog.
If some plugin or theme does not remove its custom tables you can use attached plugin. Please open it and add custom tables that should be removed with sandbox expiration.