The coffee plant is a fruit tree that gives a red-coloured fruit, called the coffee cherry, apparently because it looks like a cherry.
Each coffee cherry usually has two green coffee beans inside (the brown coffee beans result from roasting the dried green coffee beans).
A typical green coffee bean has 74% insoluble (don’t dissolve in water) components and 26% soluble (dissolve in water) components.
Caffeine is one of these 26% naturally occurring soluble components (contributes to about 1.5%) and dissolves easily in water.
If caffeine were the only soluble component, decaffeination would mean simply soaking the green coffee beans in water and removing this water after the caffeine dissolves in it.
But coffee has other soluble components as well, and those would also dissolve in the water, leaving the coffee with an unpleasant taste and texture.
The solution to this problem is to replace water with a solvent/chemical (e.g. ethyl acetate) that dissolves only the caffeine but not the other soluble components.
This is one (direct solvent technique) of the methods, but people have concerns with this one because a direct chemical (solvent) treatment of beans is involved.
In the second (indirect solvent technique) method, beans are soaked in hot water.
After caffeine & other solubles dissolve in it, beans are taken out, and ethyl acetate is added to this liquid mixture.
After some time, caffeine combines with ethyl acetate, and both of them evaporate upon heating.
The leftover liquid mix (rich in flavours but has no caffeine in it) is added back to the beans making the beans rich in flavour, but these are now caffeine-free.
In the third (Swiss water method) method, the beans are soaked in water.
This water is passed through activated charcoal, which catches caffeine (like a net catching the fish).
The caffeine-free liquid is added to a new set of beans, and apparently, when this liquid heats with a new set of beans, only caffeine gets removed.
In the fourth method (Supercritical CO2 extraction), CO2 is pressurized to change to liquid (at this stage, CO2 is called Super Critical CO2).
Super Critical CO2 tends to stick to caffeine, so when this liquidCO2 is forced through caffeine-rich water, it picks out the caffeine molecules and is pumped out.
Overall, caffeine is naturally occurring; decaffeination is done through ‘unnatural’ processes & if one is not sensitive to caffeine, one is better off avoiding decaf (also, most decaf coffees are not totally decaffeinated; some traces almost always remain).
