Taking the the Carbon atom as an example, one can analyse the bonding between the atoms more exactly to be able to understand the forming of the molecules. The reason why this is interesting, is because it has four possible bondings. It is shown above with it's six electrons as a shell-model. Thus, remaining on the second shell, are exactly these four places left for the completion.
An atom can use various methods of bonding, either to various other atoms, or to an atom of the same kind. An example of the latter, would be Oxygen, which with it's all together only two connectors, clings to another Oxygen atom. In this case, Carbon of course, has more possibilities. If it were to use all four connectors to bind to atoms of the same kind, diamond, whose hardness is almost impossible to beat, would be created.
Of course, with four connectors there are also numerous other bonding possibilities than with two, as in Oxygen or one in Hydrogen. Carbon can e.g., with six atoms, form the famous Benzene-ring or chains. The option of strengthening the chains is also given. If they are numerous, high octane fuel is developed, with fewer, Diesel is formed. The technology used to create multiple new products is called Carbon-chemistry.
In the film the scenario is shown, in which all Carbon- or more accurately all synthenic-products are removed. Not much in our normal environment is left. As far as automobiles are concerned it would be similar. Although according to the weight proportion, metals do still play a large part, synthetics, because of their lower weight, have a high volume-ratio. By the way, because of the synthetic fibres in the experiment, it's not quite clear, what we'll be wearing in the future. 03/13