It’s possible to insert uploaded images into a post.
Here are some of the images from the top post.
The drone in the top image is bigger than some of the quadcopter and hexacopters I’ve flown.
The propellers on the two 'copters are the same size. The quadcopter is a smaller than the hexacopter.
I have both of the above flying machines and I’ve been amazed how well they work.
The above flying machines are not silent but I certainly wouldn’t call them noisy. I don’t think a flapping wing drone would be quieter.
The hexacopter’s PCB has pads marked “TX” and “RX”. It might be possible to hack these machines. Even if the original board isn’t hackable, it should be possible to make your own flying machine as small as these commercial machines.
I think you’re much more likely to building something capable of flight if you use propellers rather than wings. I would encourage anyone interested in attempting to make a flapping aircraft to learn to program and control aircrafts with propellers first. The control algorithms for quadcopters and hexacopters are not trivial to program. I think the control algorithms for flapping aircraft would be even more challenging. Knowing how to control a quadcopter would be a good starting point for learning to control a flapping aircraft.
BTW, I paid $14 and $17 for the quadcopter and hexacopter shown.