Sorry, I meant a header as in where the things like the subject line, servers it was sent via, etc. are stored. It’s a part of the email that isn’t user-visible. The unsubscribe email isn’t visible to the user unless you view the raw source of the email, but email clients can use it.

I know marking as spam isn’t the same as unsubscribing. Gmail is trying to suggest unsubscribing instead of marking as spam which is reasonable (why mark it as spam when you could just unsubscribe and never get it in the first place?)