lil_satan's picture:
It's annoying that they said the PCB is "open source" at the start of the video but didn't provide a link to design files, even after being asked in the comments. oshpark (the company that manufactured the PCB for them) does have a way you can share your PCB design, which anyone else can order:
https://oshpark.com/shared_projects
but there are hundreds of shared projects so that would make it difficult to find theirs without a link, especially since we can't be sure they even did bother to share their design (it's an optional feature).
The only thing special at all about that board is that it's fairly compact. If that's not a required feature for your application you can find many other ATtiny breakout boards that will work just as well, if not better. Here's a very reasonably priced ATtiny board sold by the person who administrates the ATtiny Arduino hardware package you will likely be using to program your board: