Let me start by giving you a brief description of what the decomps are.
They're projects where the code of the different mainline GBA Pokémon games was dumped in a programming language called "
Assembly" using a disassembler, then it was translated to a different programming language, one that is easier to work with that is called "
C", and was then passed through the same compiler Game Freak used afterward, to verify that the code written was functionally equivalent to theirs.
As a result, the projects manage to recreate an exact matching copy of the games' ROMS.
Now, these projects make use of certain toolsets to do their work, such as Git and GNU Make.
The one I talked about before, as anyone could tell, was Git.
If you don't have an environment to build these projects set up, that'd be the first thing you'll want to do.
For that, you can check either
my tutorial about this, or
Pokeemerald's own INSTALL.md document.
After that, assuming that you understood what you read in either of those documents, you can come back and read my previous post again.