Residual sweetness is unfermented sugar left behind in a finished beverage at packaging — sometimes an intentional stylistic target, sometimes an unintended sign fermentation didn't go as far as planned, and the mechanism behind it differs meaningfully depending on whether you're talking about beer or cider/mead.
Beer usually controls it through mash and yeast, not by stopping fermentation
A beer's residual sweetness is normally engineered ahead of time — through mash temperature (a higher mash temperature leaves more unfermentable dextrin behind) and yeast strain selection (lower-attenuation strains finish higher) — rather than by interrupting an otherwise-complete fermentation. Sweet Stout is the clear exception on this site, using genuinely unfermentable lactose specifically because standard techniques alone can't reliably leave that much sweetness behind on their own.
Cider and mead more often stop fermentation deliberately
New England Cider and many mead recipes reach their target sweetness by deliberately halting fermentation before it finishes naturally — through cold-crashing at the right gravity, or through a proper stabilizing addition before back-sweetening — since apple juice and honey must both ferment close to fully dry left alone. Adding sugar to a batch that hasn't been properly stabilized first risks restarting fermentation in a sealed container, a real safety issue rather than just a flavor miss.