Classic is missing some directories like Buttons, Common, Sidebars, etc. And some of the TPL files to modify aren't there.
Files, unless overidden, are not found in a template such as classic...
Familiarize yourself with the overide system and creating your own template as one really does not want to use classic as all your customizations would be lost on an upgrade

https://www.zen-cart.com/tutorials/index.php?article=36
https://www.zen-cart.com/tutorials/i...hp?article=346
https://www.zen-cart.com/tutorials/index.php?article=38
https://www.zen-cart.com/tutorials/i...hp?article=142
https://www.zen-cart.com/tutorials/i...hp?article=232