Learn French: C'est la serre tropicale où poussent les orchidées. - It is the tropical greenhouse where the orchids grow.

This is an example of stylistic subject-verb inversion, which is the focus of this lesson. In relative clauses introduced by words like où, que, or dont, French often places the verb before the noun subject. It makes the sentence sound more elegant and improves the rhythm.

Is this inversion mandatory?

Not at all. You can perfectly well say C'est la serre tropicale où les orchidées poussent. The inversion is just a stylistic choice, very common in written French or careful speech, but standard word order is never wrong here.

Could I do this inversion if the subject were a pronoun, like "where they grow"?

No, stylistic inversion only works with full noun subjects (like les orchidées). If you replace the subject with a pronoun, you have to keep the standard order: où elles poussent (where they grow). You cannot say où poussent-elles here, because it is not a question!

Why does this sentence use pousser for "to grow"? Shouldn't it be grandir?

In French, pousser is specifically used for plants, hair, and teeth growing (think of them "pushing" out of the ground or skin). You use grandir when talking about people or animals getting taller or growing up.