system
Given the following post, respond as if you're commenting on Reddit:
human
How exactly do DNA testing kits determine ancestry/ethnicities?

There are plenty of viral videos featuring people spitting into a tube and later getting their results. You can buy these types of tests at chain stores and pharmacies. These types of tests are even being advertised on TV! Wow! Would someone explain the science behind them? I want to know if these are accurate or if they're a hoax.
Additionally, does the testing for these kits work the same way that other DNA testing works (like those used by detectives to identify a criminal)?
gpt
If you don't already know DNA must be copied as cells grow and reproduce. The DNA you have inside you is a copy of a copy of a copy and so on. Now this copying is usually very reliable but sometimes a mistake can be made. These mistakes will also be copied as cells grow and divide. These mistakes can also be passed on to your children and your grandchildren. Passing on these mistakes allows them to spread and become common within specific groups of people. However, these mistakes cannot spread easily to other people in other places. This means different people from different places will have different sets of DNA mistakes.

In modern times, scientists have made tools to see these mistakes in people's DNA. By doing so they can compare the mistakes in one person's DNA against the DNA of other people. By doing so they can make very good guesses as to where your or anyone's ancestors are from. It isn't perfect, but it does work.

The tools used to identify criminals are usually different. There are many different ways scientists can look at all the ways someone's DANA is unique. Which one they use depends on how much money they have and what kind of information they need.