what are they called? and how are they used?

[BrianG]

Use whatever size will work - generally just big enough to get the wire through it. Too big and it is clunky and doesn't stay wrapped. One loop is fine, two might shorten the cable too much to reach the receiver. To keep the cable from chafing against the edges, I then use heatshrink over the assembled ferrite ring to keep things in place.

And yes, the "servo" wire from the ESC to the receiver.