A common approach for beginners (and even some commercial outfits) is to add sugar to up the alcohol content of beer. Adding sugar (no need to wait until the end of fermentation) will certainly make the beer "stronger" unless so much is added it kills the yeast! Many beer yeasts will not tolerate more than 5-7% alcohol (some specialist ones may go to 10-12%).
But adding any appreciable amount of sugar makes for lousy beer! The beer will be "thin" (watery) and taste nasty (some say "cidery" which is rather putting down cider!). And be very sweet if the sugar was overdone. Some kits already add too much sugar so adding more never does any good, but modern beer kits (some malt only) are a huge improvement over several years ago when "home-brew" got a reputation for being awful.
Beer is made from malt, not sugar.