Several factors, really. Alcohol is made when yeasts eat sugars and crap out alcohol and fart CO2. Not all sources of sugar are the same, and not all sugar is fermentable. Not all yeasts are the same, either. Just like humans have selectively bred crops and animals to serve specific functions, we've (mostly unknowingly until recently with advances in microbiology) selectively bred yeasts. Some have higher alcohol tolerances than others, for they'll usually keep reproducing during fermentation until they make their environment so toxic from alcohol it kills them - and then we drink it.

Aside from that, things like water chemistry (dissolved minerals, acidity, etc.) Factor into healthy yeast reproduction (hence alcohol production). Water is the main ingredient in all alcoholic drinks, and it's ratio to fermentables usually correlates to alcohol percentage. Grape juice is packed with natural sugars. Cider and grain? Not so much. But there's different kinds of grapes, apples, and grains that all affect that fermentable sugar content. There's a whole lot of grain types that are used more for how they affect flavor than alcohol content: crystal malt, roasted barley, etc have been kilned which caramelizes some to most of the sugar in the grain. This renders the sugar unfermentable but gives the beverage certain flavors. Wheat, Rye, and Oats also don't add much sugar, but they do affect mouthfeel.

Lets do a better ELI5

Yeast are voracious eaters and farters. They want to eat sugar all day and fart out alcohol and CO2. To make beer/wine/cider, you put yeast into a sugar-water substance. For beer thats a "tea" made of grains, for cider, its made of fruit, for wine, its made of grape juice. The yeast eats up all the sugar, until it runs out, you stop it, or there is so much alcohol that the yeast dies from the alcohol (seriously).

Different beers, wines, and such will actually use different yeasts that give a different flavor and are more or less tolerant to how much alcohol they can take. The yeast used for wine can often take a bit higher alcohol.

Now, the process. Well, if you leave either to ferment more and more, they will get more and more alcohol. Wine is often fermented (and aged) for much longer than beer, leading to more alcohol, in addition, some of the wine evaporates, increasing the overall alcohol of the remaining liquid.

Lastly, wine is traditionally made 10%-15% alcohol, and served in smaller glasses. Beer is traditionally served in larger quantities, and meant to be "easier" drinking, and often 5% alcohol. Both brewers and wine makers will be targeting a specific alcohol percent for taste and flavor, a good brewer/winemaker will actually hit their target (its not as easy as it sounds!).

Fun though, there actually are some wines that come in around 8% and of course specialty beers that can be 10%-20%! Additionally, there are craft brewers experimenting with using wine yeasts in beers too, although its often not a great success.

Import export and naming laws?

Because it just does. The mash has more sugar before fermenting, so converting more sugars to alcohol during fermentation.