The data files in the game are contained in big files with a .bar extension.
To view the files you will need to use an external tool https://github.com/AOE3-Modding-Council/Resource-Manager. Note that this tool is not officially supported! For context, this is a tool used to view very large files within the game. On the Git Hub page in the ReadMe it is described as "Utility for viewing, creating and extracting files from Age of Empires III .BAR archive".
The data files can be modded by supplying a replacement file with the same name and folder structure within your mod.
However, the recommended way of modding data files is to use the additive mods system. Additive mods are merged with the base data files so that the mod doesn't (usually) need to be updated whenever the base file changes when the game is updated.
See the guide to Additive Data Mods for more details.