The use of 'het zijn' (= they are) is a peculiarity, specific to Dutch.
Instead of writing a separate grammar post on this, I'll provide you with this external link which explains it well. Below you can also find a brief explanation. :)
We use 'het zijn' followed by a noun, where the English use 'they are'.
- "Het zijn dieren." = "They are animals."
- "Het zijn aardappelen." = "They are potatoes."
- "Het zijn lieve hondjes." = "They are sweet, little dogs."
Now, when we are dealing with a an adjective this formula isn't used. Instead, you'll find sentences like these:
- "Ze zijn mooi." = "They are pretty."
- "Ze zijn aardig." = "They are nice."
- "Ze zijn groot." = "They are big."
Thank you for clarifying this, too bad I couldn't see this before I answered. I didn't get it at all.
In het zijn inheemse planten, inheemse planten is a noun phrase. So het is a type of endophoric referent (cataphoric reference, the construction het zijn as 'they are' is use as cataphoric reference).
In ze zijn inheemse, inheemse is an adjective in predicative position, and as ze it refers to something 'outside of speech', 'in the real world', so there's exophoric reference. When there's exophoric reference we use 'ze zijn'.
Hope this helps.