The Classic block mimics the function of the classic approach to editing your WordPress content. This block is created automatically when you create a page or when editing a page migrated from the original Apollo theme, but the classic block is not one that can be chosen through the Add Block function.
Content in the classic block will be available for editing through the classic and block editing experiences. You can also have WordPress automatically brake out the content in the Classic block into separate blocks according to the type of content within it by selecting the block, then clicking on the vertical dotted icon at the top of the screen and selecting the “Convert to Blocks” option.