[Singollo]

The Silmarillion was mostly written long before LotR and The Hobbit.

Actually, that's not technically true. The most basic forms of the legends found in The Silmarillion (and Unfinished Tales) existed before LotR, yes, but the final Versions of the stories were not created until after LotR (long after, in most cases). Those final Versions differed considerably from the pre-LotR Versions, too.

The pre-LotR Versions of the stories are available, in the History of Middle-earth series of academic/historical novels written by Christopher Tolkien. They make for very interesting reading, too. In fact, some of the more fleshed-out Versions of tales only exist in the HoME; Tolkien died before he finished the final detailed Version of the fall of Gondolin, for example, and Unfinished Tales only brings the story as far as Tuor's emergence from the Gates. A complete Version of the Fall is found in The Book of Lost Tales, Volume II (I think it's volume 2), and although it is much earlier and very different from what the final Version of the story would have been, it does offer some insights into where Tolkien was going with the story.