Fermentation process: yeast consumes any sugars that are present and converts them into alcohol.
wine: sugars come from harvested grapes hard seltzer: comes from straight-up fermented cane sugar. It could also come from malted barley, although technically that would make it a flavored malt beverage like Smirnoff Ice. Most alcohol seltzers are gluten-free, but if you see the words "malt," "barley malt," or "malted" anywhere on the label, it has gluten.
The alcohol content of most Hard Seltzer = spiked seltzers falls in the range of 4-6% alcohol by volume (ABV) — about the same as light beer