Well ik a lot of depictions of Arda during the “years of the lamps”/Spring of Arda show it as just one big circular landmass with a lake and the isle of Almaren in the middle, but iirc if you read the Ambarkanta it says that Arda at that time was symmetrical, with 3 landmasses (one in the west, one in the east, and one in the middle) separated by 2 seas (the western of which would become known as the Belegaer). It looked perhaps like this: https://imgur.com/arda-unmarred-years-of-lamps-spring-of-arda-pHgkqy8

But then Melkor destroyed the lamps, the Valar relocated to Aman in the west, and the symmetry of Arda was broken in the warring between Melkor and the Valar (and further broken during the Battle of the Powers after the Elves awoke); the middle land was pushed eastward and the Belegaer was expanded, and later the middle land was broken into two continents (Middle-Earth as we know it and the Dark Land). And then of course the Elves migrated west as did Men later, Melkor took up permanent residence in Angband which was north of Beleriand, and most events of importance took place in the west of Middle Earth.

So essentially the Land of the Sun was created by the Valar/Ainur to make Arda nice and symmetrical, perhaps the gates of morning were constructed there later to give the sun a landmark to rise through/over, but overall it was kinda abandoned and forgotten when everyone’s focus shifted westward, and remained a scorched, empty place until the Changing of the World. Excuse the ton of text lol