The alcohol level of cask strength Whisky is usually around 60% and this is then reduced to around 40% to be bottled and sold.
40% ABV is the legal minimum that Whisky can be to be sold.
Glenfiddich, Springbank and Bruichladdich have their own bottling plants. All other distilleries send their malt away to be bottled off site and here it is diluted with purified mains water, which is perhaps contrary to the emphasis that many place on their water sources.
Some Whiskies are also chill filtered before they are bottled. This is the process of filtering the Whisky through a miniscule mesh at 0°C, which captures flock that cloud Whisky when it becomes too cold. Flock causes cloudiness and chill filtration stops this.
Most packaging will tell you whether or not a Whisky has been chill filtered, with many brands choosing to go against the process as a way to premiumize their offering.
They are truly fascinating to watch and to see how the magical liquid we know and love goes from cask to tank to a bottle that has been spirit washed before being labelled, auto-packaged into cases, palates and than stored into the warehouse for shipping.
After bottling, Whisky is shipped all across the globe, where it is universally loved and enjoyed by me, by you, by millions of Whisky enthusiasts.