Resident Evil Requiem Ending Explained, And How It Sets Up Resident Evil 10 + DLC

Raccoon City's past and present finally come to a head in Resident Evil Requiem, which finally answers a lot of the burning questions that those trailers have hyped up. More than that, however, is that it shows the potential future of the franchise and even sets up potential DLC (but not the kind you might be expecting). We'll get to both of those in due time, but first, how does Requiem end? What is Elpis? Why is Grace so important to it? What is the secret of Raccoon City that's been hidden all this time?