Learn Dutch: Wij variëren de plantaardige voeding, naargelang we dat nodig vinden. - We vary the plant-based food as we find it necessary.

Because naargelang (depending on / as) is a subordinating conjunction. Just like omdat or hoewel, it introduces a subordinate clause and forces the finite verb (vinden) to move to the end of that clause.

What is the exact nuance of naargelang compared to a word like als (if/when)?

While als just sets up a simple condition, naargelang implies a sliding scale or a dependency. It translates best to "depending on" or "in proportion to". It shows that the amount they vary the food scales directly with how necessary they find it at any given time.

Why do we use dat instead of het to mean "it" in we dat nodig vinden?

When referring back to an entire preceding action, situation, or idea (in this case, varying the food), Dutch frequently uses dat as a demonstrative pronoun. You could use het, but dat is very common and sounds a bit more emphatic, pointing clearly back to "doing that".

Just as a refresher, why does plantaardig get an -e added to it?