To add a code, click on the Add Block button and select the Code block. Alternatively, you can start typing
/code in a new paragraph block, then press enter.
Every block comes with unique and block-specific controls that allow you to manipulate the block directly right in the editor. The code block is pretty straightforward so it only has the more options control.
Clicking on the cog icon next to the publish button will toggle the visibility of this panel. While most blocks have specific options in the editor sidebar in addition to the options found in the block toolbar, the code block does not have extra options.
The advanced tab lets you add a CSS class to your block, allowing you to write custom CSS and apply styles to the block as you see fit.