Idk about American kickboxers, but muy thai kickboxers tend to have very good punching technique. They dont just use a jab as a range finder, they can string together very long combinations. Their guards often high and tight (which can leave you open for body shots) but that's because they need to look out for kicks.
They look more mechanical to me and lack a proper hook. Thai boxers are a different breed though. There technique is different but there deadly at what they do.
Definitely different. I have been boxing since I was a kid but starting kickboxing and Muy Thai over the last couple of years. Basic punches are thrown the same but with american kickboxing or at least how I have been taught there is an emphasis on keeping range and that is mostly due to creating space for kicks. There is some infighting but you have to remember you are defending knees and elbows as well as punches. Muy Thai is different from american kickboxing in that you are not on your toes as much and fight more flat footed with a strong base and high guard.