I am studying german online via youtube, and in a lecture video about basic sentences, there were 2 sentences that baffled me based on my novice german grammar knowledge.
Das sind Unterlagen.
These are documents.
Die Unterlagen sind wichtig.
The documents are interesting.
In the first sentence, the article is neutral, whereas in the second sentence, the article is feminine. Is there any rule about sentences in German that changes the gender of an object based on their use in a sentence? If not, then what is the correct gender of the word "Unterlagen"?