What is the difference between (be) covered in & (be) covered with?

[أ/ أحمد البسيونى]

What is the difference between (be) covered in & (be) covered with?
The Longman Dictionary of Contemporary English provides an answer.

When cover means “hide or protect,” use with:
She covered the child with a blanket.
He covered his face with his hands.
The dictionary also gives an example with the preposition by: She wore a ... dress partly covered by a thin shawl.

When cover means “form a layer on a surface,” use either with or in:

The eruption of the volcano covered the town with/in fine ash.
So the preposition to use with the passive (as in your question) depends on the meaning of the word cover in the sentence.