How accurate are DNA test kits and how do they work?

For example, I was born in Iran. Both my parents are Iranian too. There are a lot of dark-skinned people from one of my parent's side and my mom often said that we probably have African genes somewhere.

I wanted to take the matter seriously.

Some DNA kits such as My heritage, 23andMe and ancestry would be the most known. Unfortunately, I'm a student and can't afford expensive kit.

Now how accurate would myheritage be, for example?

Is there also an error rate where countries/regions are displayed incorrectly?

And how does it actually work, since we are all immigrants, so to speak, and everyone has migrated at some point and produced a child with a different region.

I can't explain it well, but how far back do you look? If someone in my family was a nomad, how do you check whether they were French, for example, and how do you determine that this person comes from this or that place.

I would be very grateful if someone could enlighten me on this topic. Thanks for every answer!

Fun Question:

I live in Germany, when I kiss a European woman the saliva is transferred. They say that the saliva stays in the mouth up to years. How much would it affect the test? since kissing results in mouth-to-mouth DNA carryover