Calla Lily is used in the Genshin Impact for making seafood lily soup. It usually grows near water sources such as beaches and rivers, which are abundant in the Mondstadt region.
This plant can be found throughout the Mondsstadt region, but it is best to head to Springvale south of Mondsstadt. After that, go to Dadaupa Gorge, which is southeast of Springvale, and then go even further south.