The best thing to do is get yourself
Notepad++ or similar, and start opening the text-based files and looking at those. Anything with "cfg", "eqp", "sns", "upc", "upcge", "mis", "tsr" and a few others are actually text-based files that can be read easily. You could also download other mods, and open the same files in the mod, and compare them to what is in the stock game. Another way to do that is to use
WinMerge to compare the files. The graphic interface there really helps understand where the differences between files are.
Once you think you know what you want to attempt to tweak, you'll want to make a mod. To do that, navigate into the game folder, and if it is not there already, create a folder and name it "MODS", which is the folder that [url=https://www.subsim.com/radioroom/downloads.php?do=file&id=5473]JSGME[/url} uses to add mods to the game. Build your mod folder structure just like the game, with a named folder for your mod at the top, such as "MyMod \Data \Cfg \Main.cfg" or whatever the folder structure is. Then you can activate your mod with JSGME, and if it fails (most first attempts do), then you can back out your changes with JSGME, and try again.