Most older bikes with cables for the throttle (one pull cable, one push cable) are all very similar.
Remove the 2 housing screws near the rusty throttle tubes, at the bottom of the housing.
When you lift off the top half of the housing, you will see where the 2 cable ends attach to the throttle tube (with its attached rubber throttle grip).
You rotate each of these throttle cable ends 90 degrees so they slide sideways out of their mounting holes at the end of the throttle tube.
It usually takes 3 times longer or more to put the cable ends back on, put the housing back together and adjust the throttle for a little cable slack (not too much or too little cable slack).
The rusty throttle tube that is partly unscrewed should be screwed in all the way before you adjust the cables- the adjustment is at the other end of this tube (a long rotating adjuster and 2 locknuts, one at each end).
You only unscrew this rusty tube from the housing to replace the cables and/or the housing, not for adjusting the cables.