trying to keep it simple :) ...
the RF current is constrained to the centre conductor, more so to the outer skin of that conductor. RF currents have a skin effect traveling down a conductor that DC currents dont. ie ... DC current will flow through the complete conductor where RF currents flow on the outer surface, only penetrating into the conductor minimally. the higher the freq, the greater that effect (shallower the depth).
tho there is some EM field around the centre conductor, the conductor is carrying the principal current, not really acting as a waveguide. There is no bouncing down a waveguide, when referring to a coax cable. In fact that applies for the waveguide as well :)
Yes, the EM fields are kept contained inside the tube, but the RF currents are flowing along the metal walls
In a waveguide, there is no centre conductor...
A wave guide is a combination of a twin lead transmission line and 1/4 wave metallic insulators. In a rectangular waveguide the RF currents down the centre of the 2 long sides, commonly called the H plane.
here's my drawing taken from one of my amateur radio books some time ago
I admit upfront, I'm not an artist ;)
https://www.physicsforums.com/attachment.php?attachmentid=36035&stc=1&d=1306749618
so a waveguide is basically 2 parallel conductors held separated by an infinite number of 1/4 wave stubs ( that is.. a solid metal wall)
hope that helps a bit :)
Dave