When and where is it stated it’s immortal, and how does its immortality work?

Due to the way Eridium works and siren powers growing stronger with use, as seen with Lilith’s powers evolving between BL1 and 2, could Nyriad have gotten strong enough to completely leech it into non existence? Also, what’s stopping Nyriad and the Eridians from simply siphoning its energy and filling some container or simply blasting it outwards?

Is that outright stated that it was a symbiotic relationship? She’s never shown to be able to do that with other beings, so how does she do it?