Very similar to how all Coca Cola in the world tastes the same despite Coke not actually producing any themselves.
Coke ship out the postmix syrup to licensed bottling plants all over the world, the plants bottle the coke to a standardised recipe using their own water supply (probably distilled).
"Guinness Flavour Extract, a dehydrated, hopped
wort extract made from barley malt and roasted barley, is used for overseas production of the stout. The syrup is shipped from Ireland, where it is added at the ratio of 1:49 to locally brewed pale beer. In most overseas markets, Guinness Flavour Extract (GFE) is blended with locally brewed beer to produce FES."
To clear up some misinformation Guinness Draught isn't produced in Nigeria, they produce Guinness Foreign Extra Stout among others.
As far as I can tell (without actually phoning Deagio customer services) Guinness Draught in kegs is only produced in Ireland and is shipped out exactly the same.
All the other versions seem to use the wort extract and water it down with local PA to a standardised recipe.