There are a number of electrical devices in the torso and above that need power to operate. A battery or power supply could be mounted in the torso to do this but in general, to keep the center of gravity low, its a good idea to keep anything heavy like batteries down low in the tread section. If your torso is going to rotate only a small amount, say 1/4 of a turn, running suitable good quality stranded wire from the tread section to the torso should do the trick.

But if the torso is going to be able to turn much more it may need some way of sending power or signals from the stationary parts to the rotating parts. This is nothing new, machines have been around for years that do this. They are often called "slip rings". Slip rings are nothing fancy: just a rotating ring and stationary brush. Brushes can be solid graphite or flexible copper, rings are often copper alloy. Sometimes more exotic materials like gold, silver or mercury are used. There are many variations depending on power level and rotational speed required. Either power or signals (or both) can be transmitted across slip rings.

For a B9 robot torso that has a rotational speed of maybe 3 or 4 rpm nothing special is needed, many different designs will work. But to be economical slip rings can be made from commonly available parts, just junk that I had laying around the garage. And actually there are millions of slip rings in use today, its almost certain that you own several. They are almost always used in car alternators.

This is a combination voltage regulator and brush assembly from a Bosch alternator from an old VW I once owned. It was simple to remove the black part above, the“regulator”, from the brush assembly. This brush assembly is then mounted to a stationary object, in this case the housing of the motor that turns the torso.

While it appears possible to buy/find the slip rings themselves these parts are not often available. Its not that difficult to make them, a short length of plastic rod, some rings cut from a copper pipe coupling provide the raw material.

The red and black wires attached to the brushes run down to the batteries in the tread section. Just visible soldered to the copper slip rings are more red and black wires that go up (down in the picture) to the rotating torso.

Of course it is possible to make an assembly with more rings to have more conductors. For the slightly different robot 2 conductors should be adequate, they will send 24 volt DC (at least 10 amps) from the batteries in the bottom of  the tread section up to the torso. A 24 to 12 volt converter identical to the one used in the tread section will allow both 24 and 12 volts to be available in the torso.