Are those legit? I mean, I found a few ones but I just don't understand the tech behind it. How do they know what happened to the car?

I use FAXVIN and they have all the information you may need on the site. But the previous comment basically is correct. Oversimplified but correct. Again, if you want to know everything, you'd better prepare to spend a while reading about it. And speaking of trust, yes, those tools are reliable and you can trust the output.