First there is a major difference between ethylene and ethane.
Ethane has little value compared to ethylene. Ethane can be found in natural gas or LPG. Ethylene is very valuable, because it is a molecule that is used as a building block for many other chemicals.
You should research "cracking". A simple definition is: large molecules are broken into smaller molecules.
Air is excluded, because cracking takes place at high temperature and pressure. If air is at a high enough concentration the reactor could explode. Additionally, air is mostly nitrogen. At high temperature and pressure nitrogen will react to form unwanted byproducts and contaminants.
Typically, if you want to produce ethylene. Lighter hydrocarbons are used with steam cracking. Catalytic cracking is normally used to produce diesel and gasoline. Ethylene can be produced this way, but my understanding is, it is not the preferred way.
The higher the temperature, the smaller you can crack big molecules to smaller molecules. Lower temperatures will produce more propylene, C4, and larger molecules. Propylene does not have the same value as ethylene.
Ethylene can be converted to ethylene oxide, another major chemical used as a raw material.