Castellation is used to make the connections:
The pcb itself is using pcb art technique to give the shape of a tree covered in snow and some snowflakes and ball ornaments.
To help making the programm for the ATtiny a helper website is made to make the led addressing easier:
All files including the KiCad project is hosted on GitHub: https://github.com/designer2k2/2d-xmas-tree
And to help with assembly a video: (in german, with english captions, turn them on)