Why don't you read the article?

literally the first paragraph:

> Duranium is an immutable variant of postmarketOS, built around the idea that your device should just work, and keep working. You shouldn't need to know what a terminal is to keep your device running.

“The package base for Duranium is shared with current versions of postmarketOS, and improvements flow into both. Think of it as a different deployment model on top, not a fork.”