I don’t really know how they come up with these numbers

They sample multiple people from a given region of the world and then look at possible genetic similarities between most individuals in that sample.

Then, they collect your genetic data and “match” to all the different signatures they’ve collected from different regions, and compute a similarity score.

In theory, if they had sufficient samples and the genes were very characteristic, this could work. In practice, any geneticist will be able to point out multiple flaws with this methodology.

There are indeed certain traits that only occur in specific populations… And while someone else totally unrelated could randomly have a similar mutation, it’d be unlikely. But those are rare, and absolutely not something that can be used to say “78% German”