There are different types! The really big ones that is for outside and smaller house friendly.
Usually it is used with peat it can be with kind of tap to 'flush' it automatically or doing it manually. As well to avoid smell house friendly ones has air pipe and liquids pipe.
They don't really make bad smell, some of them is up to a month for 4 peoples. Some campsites are using it and it is for bigger traffic and now it is really becoming more common even in the living houses more rural of you have where to use it afterwards.
I had never own do it but used and of it is just simply maintained it is not having issues. Oh just it is not good to put food leftovers. Some have buckets inside so it is easy to cl an up.
I am kind of 'poop storage' expert 💩