What is the scoop with surface area vs cross-sectional area?

I've always thought the electricity in a cable flowed through the entire cable, but several posts I've read state electricity only flows over the outside surface of a cable. (Under the insulation I assume.) If that is true, why don't cables have a flexible core of something other than metal strands? It would make them lighter and cheaper. Are the individual strands making up a marine cable considered multiple cables, each with their own 'surface area'?
With the Epoch 12V 300Ah V2, and this applies across the board, Can is short for Controller Area Network Bus, so they are one in the same.. Those ports are primarily intended to communicate with Victron hardware to display multiple batteries. With or without that hardware, they will functionally communicate just fine, but you won't be able to monitor them as a bank. Rather, they will be displayed on the Bluetooth app as individual units. The ideal cable to use would be
a CAT5. Multiple batteries can be monitored via Victron Cerbo/Ekrano. The batteries do not require direct connection to each other. The differences between ethernet cables and RS485 cables are the pinout, shielding, and twist.