First, you’ll need a resource to read the .BAR files. This one works very well. [v.0.4] Resource Manager - Viewing, comparing, creating and extracting files from Age of Empires III .BAR archive
The .BAR files you will be interested in are located in the UI folder (file path similar to: C:\Program Files (x86)\Steam\steamapps\common\AoE3DE\Game\UI). The UI.bar file contains files that control the layout of the pages while the UIResources<1 through 4>.bar files contain the .pngs that can be referenced.
Once you locate the file you want to modify in a .BAR (probably MAINMENU.XAML and MAINMENUOPTIONS.XAML), extract them to your desktop. This should give you the files nested in the correct folders to include within your mod. If it doesn’t, you will need to keep the same folder structure as indicated in the File Name in your BAR editor. If you are adding images, consider keeping a similar file path to ones you will be referencing.
Now create a new folder with a title for your mod in the local mods folder (file path should be along the lines of C:\Users<username>\Games\Age of Empires 3 DE<randomnumbers>\mods\local). Drag and drop the folder you downloaded to your desktop here. You can now activate the mod when you start up AOE3DE (make sure to restart after activating)!
As for editing and actually making changes, you can use Notepad or Notepad++ on the XAML files. I would recommend adding .png files instead of editing them since they may be used in other places. Heaven Games has some resources that should help with understanding some of the commands, but it seems like you’re mainly interested in layout.
Good luck, I’ll try to help if you have questions!