How to exercise this part of my arm?

As you can see in the image, I'm not the healthiest guy. I have been doing pushups recently and it's gradually boosting my part "A", but part B & C (the wrist) have no effect. Specially the wrist, it looks like I got no meat in there at all, only bones. Is there any exercise or anything that would help me build some muscle structure in part B & the wrist? I eat healthy foods properly but they don't work on my hand, it seems. Advice? Thanks! :)

https://i.imgur.com/Bd950Cl.jpg

https://i.imgur.com/1ngmXsi.jpg
How to exercise this part of my arm?
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