On current 12-speed Shimano Di2 road groups, the handlebar shifters communicate wirelessly with the system. That removes the need to route wires beneath the bar tape. The front and rear derailleurs, meanwhile, connect by wire to one battery housed in the frame. Electronic shifting therefore does not mean a separate battery on every component.
The layout also changes how gears can be selected. The two derailleurs operate as a coordinated system: in Shimano’s synchronized mode, the front derailleur can shift automatically as the rider changes rear gears. The E-TUBE settings let riders choose where that front shift happens, while manual shifting remains available.
For the rider, the important difference is in the controls. A button press no longer pulls a cable directly; electronics move the derailleurs and can coordinate both sets of gears. Before a longer ride, it is still sensible to check the system battery and whether a particular groupset supports the settings you want.