while there's a lot of interpretations of what is a wrist on a fantasy creature, we have pretty obvious anatomical analogies we can turn to that make it way easier, and undel has explained what she determines the "hand/wrist" of a dragon's wing is before in her books. she equates them in design to arms, sometimes quoting it as "human arms". our own interpretations be damned, there's documented evidence of undel's interpretations, too, and what she says deviates from what she's taught others in her own professionally produced/edited/sold writing, which she still uses as a foundation of her current art.

https://imgur.com/a/iGHhFxJ

i'm sorry for those bad images; i don't have her books anymore, so i had to source these from tumblr/amazon. you can even preview her books on amazon to find some of these pages, too.