All Apple Power Adapters are universal voltage - designed to operate from 110-240V at 50/60Hz AC Mains input.
The newer 18/20W Power Adapters are also USB-PD compliant - and automatically negotiate the optimum output voltage and current for the connected device. These Power Adapters default to the standard 5V USB output, but can also utilise 12V and greater with USB-PD compliant models of iPad and iPhone.
Some regional Power Adapters have AC Mains “pins” that may be region specific - whilst others have a “duck-head” that can be easily changed to accommodate the local AC sockets. All simply require a simple pin-converter, readily available, to connect the Power Adapter to a different style AC socket. Alternatively, you can simply purchase a region-specific Power Adapter in the country that you have travelled to.
I hope this clarification is helpful.