nitroexplorer on the ds is about pulling files from a file system.
Commercial GBA games feature no such thing. Some homebrew will though.
There are a few techniques for finding the binary, and for finding the sound if it uses the sappy format. Likewise compression can have indicators and things to be searched for.
If you are specifically interested in pokemon though then most mainline pokemon games end up with tools made, or at least their internals documented. I am less familiar with the modern versions of these though so you will have to go looking.