What is correct about digital signatures?

When a document is signed, a hash of its exact contents is computed and then encrypted with the signer's private key to produce the digital signature. Because the signature literally contains that specific hash value, attaching it to a different document causes verification to fail - the recomputed hash of the new document will not match the hash embedded in the signature.

A digital signature cannot be reused across different documents even of the same type, because each document's unique content produces a different hash value that the signature is bound to.

While the claim that a signature cannot be moved between documents is correct, the stated reason - that it is plain text - is factually wrong; the real reason is that it encodes the document-specific hash.

Digital signatures are not reusable user credentials issued once; a new signature must be computed for every individual document by applying the signer's private key to that document's unique hash.