A flare joint is used on gas fittings and uses a nut and flare fitting to make the connection. This is a dry fit with no joint compound. The copper is soft enough that it compresses with the nut and forms the seal.

Iron generally corrodes faster because the oxide layer (rust) does not seal the metal's surface from oxygen like the corrosion on copper does.

It won't work. There is entirely too much pressure in the cylinders to seal up a head gasket... but many people have to try it themselves.

The best sealers for copper pipe are a combination of plumbers tape and plumbers epoxy. The tape is used to temporarily seal any holes while the epoxy dries.

When soft copper tubing is used the end is flared so that it can seal against the fittings. A flaring tool is used to spread the end of the tubing so that it looks like a small funnel. When you tighten the nut onto a fitting it compresses the copper against the flare on the fitting and makes a seal. It is a dry seal, no pipe dope should be used on a flare fitting. This is almost always on a gas line of some sort, air, natural gas, propane or some other gas. It can be used for liquids, but generally isn't.