Hello @noaki,
In the WordPress admin area from the left side menus click on the Footer Builder which comes with Deep Core plugin, and then click on the Add new Footer.
After adding a new item you can edit that page with Elementor and create the Footer with Elementor widgets.
After you are done with Footer you need to set that as your main Footer for the entire site.
In order to set the Footer please navigate to WordPress -> Deep -> Theme Options -> Footer -> Footer Builder and enable the Custom Footer after that the dropdown will appear that you should select your desired Footer item and save the Theme Options, then the Footer will appear on your website.