Learn English – the common name of the soap that is put on the lavatory seat? (picture is attached)

Although what you're showing in your example is understandable, what you are probably looking for is called a

caret

and marks an insertion point for letters or words.

Strictly speaking, in the proofreading / copy editing world, the caret appears below the typed line and points upwards, and the handwritten edited text appears above the typed line

Don't confuse the clear wrapping with the dust jacket that's on a hardcover book. For most people, "cover" will mean the same thing as dust jacket.

I think what you're describing is clear plastic shrinkwrap.

A protective wrapping made from clear plastic film that is shrunk onto an object by heat

EDIT The loose plastic covering that a magazine comes in might better be called a sleeve or a mailer, since it's not shrunk tightly onto the magazine inside.