Is this posable in any way?
From my knowledge, yes but limited poses I think. I believe they come with "stickers" or they're probably masks that allows you to change the facial expressions on the face.
Yeah, it works something like that. I've never owned a Nendoroid, but I think they usually come with some alternate face pieces that you can click in, as well as alternate arm poses. In the pictures of Korra, you can see that her hand gestures are different between the two photographs.
It also looks like her legs are poseable, because you can see little joints in the photograph. (Actually, looking at it now, it also looks like she has alternate leg pieces between the first and second photograph. At the very least, they seem to have swapped the legs around.) The website says that the toy can't stand on its own, and requires the stand it comes with.
Here's what a Nendoroid box might include (2 examples, sorry for poor images):
Sometimes they just have alternate faces, sometimes they have an entire extra head. You can also swap the face plates/ heads with other Nendoroids, (so long as they are a compatible shape):
(That's Elsa in the Iron Man suit