Hello I'm trying to figure out how modular arm joints are designed and what kind of bearings/shafts are used for a modular-type robotic arm. Take "UR arm" for example. I believe those 'T-shaped pipes' include both a drive and bearing system. And as you can see from second image, it can be detached easily. So I think it's not just a simple "motor shaft connecting to the member that we want to rotate" mechanism. I'm wondering which type of mechanism and bearing system is inside of those T-shaped pipes. How can i transfer rotational motion to a member without using shafts?
Like Andy said, a Harmonic drive seems to be the most likely answer. But your final question is very vague. The other option is that it is a direct-drive system (with a clutch to protect the motor), and they are using a very high-torque motor. They certainly seem to have the diameter, as long as you aren't trying to lift something too heavy.
As for your last question, if you must avoid shafts while trying to achieve rotational motion, you're probably going to have to use a pneumatic/hydraulic system. Most companies avoid this - I know of only two that use hydraulics for rotational motion, leaks will be the bane of your existence. The other option is to start playing with some pretty advanced materials (say some electro-reactive ones, that change shape with electrical current)