Historically soldering tips were copper, placed in braziers. One tip at a time was used; when the heat had transferred from the tip to the solder (and depleted the heat reserve) it was placed back in the brazier of charcoal and the next tip was used.
Much later gas irons were in common use. These used a gas jet to heat the soldering bolt/tip. They are very fast, but require significant amounts of experience to properly regulate the temperature.
Currently, electric soldering irons are used; they consist of coil or ceramic heating elements, which retain heat differently, and warm up the mass differently, with internal or external rheostats, and different power ratings - which change how long a bead can be run.
The soldering iron used must be of a high enough wattage to readily melt the solder and be able to reheat fast enough to maintain the necessary melting temperature. The tip can't be so small it can't maintain the heat and not so big it covers much more area than wanted.
For soldering leaded panels a 100w iron with a 3/8" temperature controlled tip that maintains a constant 370°C (700° F) is suitable.
For copper foil a higher temperature controlled tip is used. This normally runs at 425°C (800°F). Sometimes a tip of ¼” is used where more delicate beads are being run. But there is little difference in the resulting bead - only that the smaller bit takes slightly longer to heat up.
If a lot of soldering is required that has sustained heat requirements, you might consider a 200W iron. These can deliver heat more quickly and evenly than those with lesser wattage.