Had a book about 30 years ago on timber joints in Japan. They listed a whole series of wood joints that required no nails and most would flex and survive a earth quake. Don't know what happened to it but it is an art form in many places. My grand dad always remarked on log cabins, those with skills had square corners and those who didn't overlapped the logs and let them stick out. Neighbor tore down an old house, first one built in area, and the log part had sheet rock on the inside and siding on the outside and was so well built that no one knew it wasn't framed. All done with an axe and an adze most likely. The one you show is unique in that it connects 3 logs or poles, not two. Interesting, thank you.
A couple vids on timber joints. Japanese Master Craftsmen Dry Fitting Huge, Insanely Complicated Wood Joints And this, might be the book referred to by @duane - https://www.amazon.com/dp/0881791210/?tag=survivalmonke-20 (Links to more books on the subject at that page, too.)
Square Lashing | How to Make a Square Lashing | Scouting Knots Requires only rope and a few minutes. I've been kbnown to notch the sides of both poles a bit to reduce any slippage.
As a quick fix, rope is easy and convenient...for more permanent structures; hardware, fixings, and/joinery techniques might be preferred. Heavy gauge galvanised wire and hoop strapping are also options to consider.
Tarring the rope, if non synthetic rope is being used, might eke a little more life out of a rope secured structure
