Firestore how to store reference to document / how to retrieve it?

You're just missing one step. This took me a while to figure out as well.

First, store a variable DocumentReference!

var userRef: DocumentReference!

Then, you want to create a pointer to the document ID — create a DocumentReference from that <- this part you are missing

if let documentRefString = db.collection("users").document(documentId) {
  self.userRef = db.document("users/\(documentRefString)")
}

Finally, resume saving your data to the database

db.collection("publications").document().setData([
    "author": userRef,
    "content": self.uploadedImagesURLs
]) { err in
    if let err = err {
        print("Error writing document: \(err)")
    } else {
        print("Document successfully written!")
    }
}

Hope that helps!