Trailers, as you might expect of something that is support tons of weight at 80 KPH, aren’t as simple as they are sometimes made out to be. There are some serious, and not intuitively obvious issues (unlike the need for extra braking and motive power for a bigger trailer)
There’s too much for me to write about, so I’ll just give you some of the links I have found:
It seems like info on trailers is kind of hard to find, and you will see that none of the above pages are thorough about the subject, e.g. the wiring page omits info about CANbus, which wikipedia mentions, while wikipedia doesn’t even say anything about the various types of connectors. Also, the explanations for e.g. the way a weight distributing hitch works is kind of dodgy, but you can see how it works if you think about it. I should try to improve the wikipedia article sometime. If you find any particularly good pages, do post in the comments.