The Learn LaTeX website uses 2 types of licenses:
The content of this website is available under the Creative Commons BY-SA 4.0 license. That means you can use the material as long as you say where it came from and let others do the same.
See LICENSE for details.
The code examples are licensed as Creative Commons CC0 (No Rights Reserved). This is a legal text that is (essentially) the same as Public Domain, at least in countries where that concept exists. The key point is that you can use the examples without any issues. You do not need to reference them, you do not need to share them when you modify them: you can just use them.