Everything in nature is spiritual. Everything. Some places more than others and some things more than others. It can accumulate in places and be more dense, like Ionia and be less dense in barren or harsh areas like Shurima and Freljord. And also be less dense in human populated areas with a lot of artificial stuff.
This "spirit energy" that exists in all natural things can sometimes cling onto shreds of magic / thought / emotion.
This can accumulate over time and they become things like Ornn, Anivia, Voli.
Also applies to demons.
It grows vines growing towards light and water. It wants to live.
Analogy for good spirits and bad spirits:
Grow a rose in coloured water and they will have petals with that colour.