The yeast takes 2 - 3 days to ferment nearly all of the sugar in the wort into alcohol, and the FV now contains beer rather than wort. The fermentation is stopped by removing the yeast from the top (known as 'skimming' the FV) and cooling the beer in the vessel.
The yeast skimeed from the beer is kept refrigerated in sterile buckets ready to be used to ferment next week's brews. Surplus yeast from big breweries is usually sold to the Marmite factory in Burton on Trent .( www.marmite.co.uk )