People carry their own water, chop their own firewood, cook over a hearth fire in small hand-made thatched mud huts, make their own furniture, grow their own food (possibly using animals to plough), prepare their own food by hand from scratch, make their own clothing, do their laundry in a stream, poop in the woods or in an outhouse, clean themselves in a sweat bath or with a damp cloth, carry goods from place to place on their backs or in simple unpowered carts, etc.
Though there are fewer and fewer such places as time goes on. For instance rural Mexico has changed dramatically in the past 2 generations.
They aren’t precisely “less developed” than 18th century Europe, but they are more crowded and often have comparable hygiene, medical care, etc. Again they often lack basic amenities like running water, sewers, and grid electricity. The biggest difference is probably that many of these have much worse air pollution than anywhere in the 18th century.
Yes, there are many places which smell potent if you are used to living in a wealthy suburb, but the smells are not really like the description above, especially after a week or two of acclimation, except maybe for extraordinarily dainty hypersensitive people. Patrick Süskind is exaggerating for effect.
I'm not sure what air pollution you're referring to though?
You seem to be implying that modern third world is so far behind so as to resemble 18th century Paris. While I agree with you that the quote in parent seems exaggerated I don't think even a modern very poor country will stink like 18th century London and Paris. 18th century was a time of horse driven transport and little to no sewer system. The main stench would be from those two things which in modern times is mostly absent even in poor third world countries.