If this is how a sill (or mudsill) is supposed to be installed then I confess to being the worst kind of hack—having only used butt joints while bolting PT lumber onto foundations. But then I was building in the American style where the sill need only be level and firmly bolted down.
What’s happening in video below is an entirely different thing. The sill is made from cedar and is joined with incredibly complicated hand-cut joints. This kind of joinery was developed more than 1,000 years ago in Japan. There were reasons for it then, some having to do with engineering and the scarcity of metal fasteners and others having to do with aesthetics.
Traditional joinery is still practiced in Japan, but on a limited scale because few can afford it. To this day, it’s not uncommon for studs to be mortised into joists (the mortises would be cut by machine at a fabrication plant) but sills joined in this manner are clearly something special. Not that I’ll ever install such a sill, but it’s cool to see how the joinery is done and wonder what it would be like to work for a client willing and able to pay for this level of craftsmanship.