Mod-enabled games allow for customization: adding new items, characters, maps, modes, and even entire campaigns created by the community. Thanks to mods, such games survive for years, and sometimes even evolve into new games. Classic examples include Skyrim, the Fallout series, RimWorld, and Terraria.
Often yes, but it depends on the game version and the mod itself. Check the giveaway description and comments — they usually explain how to install the mods.
You can download modded games via torrent: the filters above will help you choose by language, size, and build type.