Rodinia is a small square world (4000 x 4000) that was originally created as a test world for the update to 1.8. It was the first place to acquire the new stone types (andesite, diorite, and granite), among other new blocks. Once the update was ready, Rodinia was linked to the main network.
Rodinia is used by some as a "base" of sorts for people to store their items. This is why it's sometimes referred to as an "ender chest".
The spawn for Rodinia is at Point Zero.
As described above, Rodinia was first generated under 1.8, the Bountiful Update. This gave the players access to the newer materials. A major expansion of 2000 blocks North and West was done to the overworld after 1.18 in order to give players access to the new biomes.
Of note, Rodinia's Nether was expanded after 1.16 (the Nether Update) to give access to its new biomes and resources.
Point Zero
Xanthi
Heraklion
Dusk Citadel
Troezen
Miletus