system
A chat on subreddit /r/NoStupidQuestions
human
There are pros and cons to each design.  European toilets use a washdown system.  These have larger openings so less apt to clog.  They work by pushing the waste down.  They also use less water.  The downside is that they need to be cleaned more often and can smell.

American toilets use a syphon system.  They have a smaller hole to aid in the syphoning mechanism.  They can be more prone to clogging but if you are clogging it often then that sounds like you need to either use less toilet paper or change your diet/bathroom frequency.
gpt
I’ve lived in several European countries and I’ve never found a private toilet to smell. I’m not including public bathrooms, some of which were a bit iffy, but that’s not exclusive to Europe. 
We usually clean the toilet once a week and use the brush as needed (for some reason toddler poop is very sticky lol).

So I disagree with your assertion that European wcs are smelly.