The classic block looks just like the classic editor, but it’s in block form. Custom TinyMCE buttons are found in this block.

In order to add a classic block, click on the Block Inserter icon.

You can also type /classic and hit enter in a new paragraph block to add one quickly.

Detailed instructions on adding blocks can be found here.

Block Toolbar

Each block has its own block-specific controls that allow you to manipulate the block right in the editor. The classic block offers the ability to edit as HTML, duplicate, or convert to blocks :

Class block toolbar.
Classic Block Interface

Block Settings

While most blocks have specific options in the editor sidebar in addition to the options found in the block toolbar, the classic block does not have any additional options.