They were heated on stoves and in fires. The disadvantage of the sad irons is that their handles get hot. The most valuable to collectors are sad irons with unusual designs, especially figural shapes. Slug irons are heated by inserting a hot slug of metal into a cavity in the iron. These can be very valuable.
Charcoal irons are self-heating devices.
Using them is like ironing with a miniature stove. Dirt and temperature regulations were problems but they are still widely used today in developing countries. Liquid-fuel irons relied on gasoline, kerosene, alcohol and natural gas. These were a big improvement over the charcoal irons because they didn't produce soot. Introduced more than 100 years ago, electric irons were safer, lighter and didn't need constant attention.