I know what “kombucha” is. I also know it is well-known outside of Japan, but as a different drink. I believe the word itself comes originally from Japanese. “Kombu” means edible kelp, and “cha” does tea in Japanese. So, this is literally “kelp tea”.
We prepare Japanese-type Kombucha by pouring hot water over dried powdered kelp in a cup, like you do instant coffee. Also, this powdered kelp can be used as a seasoning for Japanese cooking.