The Hobbit was Tolkien writing a story and adventure - indeed it was a challenge thrown to him by C.S. Lewis to actually write a story not a lore or mythology.
Lord of the Rings is half way - its in part mythology but also a story and adventure.
Silmarillion is notes and history and lore. Indeed the bulk of what comes after LotR is of a similar nature. Tolkien did world building on a grand scale that most authors never get close to achieving (and those who do generally do it bit by bit over a 20 book series). Indeed one can argue that Tolkien - for all that he also lifted much from Norse legends, was one of the first to take world building "seriously" in writing beyond publishing a story.
His intent was to create a mythology for England. To that end he created his own language (not just taking an existing one - he built it from the ground up); his own world and lore. There's such a vast body his son spent years sifting through notes and adding and mixing them into further stories.
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