You definitely need to distinguish “electrical” devices from “electronic” devices but a safer approach is to read the plug.
A power supply (ex. Phone charger) will be stamped with a voltage range and power draw, which probably includes everywhere but you can match it against the electrical service where you are. You may need a plug adapter but a simple mechanical adapter is sufficient.
an electrical appliance (ex. Hair dryer) plug is probably not stamped with electrical requirements so the safe approach is to only use it in the intended country
No, voltage adapters are built into basically every electronic device now so it doesn’t matter which you plug into
Yes for electronic devices that expect low voltage DC and have a converter, like laptops, phone chargers, etc.
But don’t try and take a 120V hairdryer on holiday and plug it in because it will certainly blow up.
You definitely need to distinguish “electrical” devices from “electronic” devices but a safer approach is to read the plug.