A good place to start is with the original Game Boy - you can probably gather from the article that, to build a GBA emulator, you need to support some aspects of the Game Boy anyway (notably the sound chip). That would give you a simpler place to start getting familiar with the general concepts (like memory-mapped IO, graphics hardware, etc) and structure (like syncing emulation speed to the sound buffer) as a foundation before jumping into the more complicated GBA.
There's tons of tutorials[1], documentation[2], and tests[3] for building a Game Boy emulator, not to mention existing implementations in every language under the sun. Your favorite LLM has also surely already read them all, so you can have as much hand-holding as you'd like.
[1] https://www.youtube.com/watch?v=e87qKixKFME&list=PLVxiWMqQvh...
[2] https://gbdev.io/pandocs/
[3] https://github.com/c-sp/game-boy-test-roms