This is a TIL post from earlier today TL;DR Dovetail joints are logs or timber cut to fit each other. Used to join pieces of wood without the use of nails, pegs, or glue. There are famous examples of very old Japanese structures using this technique to construct buildings.
The top comment from the thread was from a carpenter. He asserts that not only was it impractical to carve such complex joints, but that anything beyond the simplest one would lower the overall strength of such a construction.
He figures that this method of construction was only used on temples. And while Japanese iron was unsuitable for nails(no source, so an answer to that too if you have one), they probably used much simpler joints or wooden pegs/dowels to hold most of their buildings together.
Edward Morse's 1886 book, Japanese Homes and Their Surroundings ( reprinted by Dover) , describes the use of elaborate joints for regular house construction. Morse also did not think that the joints were necessarily stronger than simpler ones, however, or the nailed-together Western frames. The joinery was also echoed in eastern furniture. Traditional Chinese furniture has quite complex joinery- a chair can be actually quite a puzzle, with several different interlocking parts with sliding keys locking each joint together. Whether they decided to first build houses like chairs, or first decided to build their chairs like houses, is beyond my knowledge.
Japan was iron-poor, and so you would expect nails would have been expensive...but if they were able to make swords out of the iron they could find, it is hard to imagine how they could not make workable nails from it.