The digital signature is always all-encompassing. Once it is given, it applies to all files within documents and to all signed content. Signed documents are always with you in a secure cloud service.
One digital signature can sign multiple files. It really doesn't matter where you place it on the document. Invite collaborators via email or simply copy a share link via Slack or Teams.
Every signature is cryptographically linked to the signed digital files and to the identity of the signer. Always and immutably. You don't have to remember new passwords or ask security questions to recover them.