How do steam-irons/flat-irons directly convert electricity to heat?
Nichrome wire, typically. wikipedia.org has a reasonable write-up on it.
Steam irons etc contain a large resistor which gets hot when electric current is passed through it.
This heat is conducted to the flat plate by metals which are good heat conductors but the electricity is insulated from the rest of the iron by a non metal which is a good electrical insulator (but a good conductor of heat.)
You can calculate the heat power generated.
The unit is the WATT.
Power = voltage times the voltage, divided by the resistance of the resistor.
For example if the voltage was 110 volts and the resistance of the resistor was 12 ohms, the heat power generated would be 110 * 110 / 12 or 1008 watts.
Separate names with a comma.