Smells are the volatile or airy components of things. It is very similar to tastes as they are the denser more closely experienced forms of tastes perhaps.
If something is heated, it volatilises or turns into a gas form or releases an air which can be smelt. If food items are heated, their most strong smells volatilise first.
Then after that you only get a subtle smell from them as I assume the physical substance is transmuting from physical to liquid to air which allows you to smell it. Subtle oils have mostly gone which contain the majority of the smell.
To extract a substances smell, the essential oil or subtle volatile oil is distilled. There are different processes like using lard or just mass distilling flowers in water. These are then sold in small bottles, usually mixed with common oils or alcohol by the name "essential oils".
The nose and mouth processes smell. Volatile substances can be received by electrochemical sensors that are vibrated by the gas.
Smell sensing technology is a very possible thing in the current age with polymer, chemical coated quartz oscillators in an array. Each sensor receives a different kind of smell and then the digital signal is processed into libraries of smells virtually to categorise them.
Smell creation is still undeveloped as far as I know. Electrodes can be placed on the internal nose nerves to stimulate them, though this is an invasive technique.
Chemical smell creators/actuators exist which spray essential oils in a volatile substance like alcohol in order to mimic smells. This is similar to how perfumes work.
Spicy - Cinnamon, pepper, chilli and clove. Stimulating, hot, burning smells.
Sweet - Honey, flowers, fruit and candy. Stimulating, enticing, relaxing and desirable.
Earthy - Dirt, faeces, bark and patchouli. Grounding, contracting, drying and heavy.
Watery - Rain, water, steam and mould. Relaxing, expanding and fresh.
Salty - Fish, salt and urine. Savoury, grounding.
Plants - Wood and leaves, fresh, bitter.
Mineral - Mild, old, dusty.
Chemical - Sweet, strange, sharp, spacey.