Actually, both worlds are in sorted in across all three tilesets, organized in sections depending on their usage. Feel free to explore them and to learn how it works.
Otherwise, finding and "add[ing files] to the project" or "import[ing] from another poject" isn't that hard, and there are many turorials on how to do this.
So good luck with your project (and feel free to ask if you need help, there will alwys e someone to answer or give you hints).