Side quests in Diablo and bg3 are vaaaaastly different. They're much more like Skyrim but that still hardly does it justice. In Diablo you find a burning building then the survivor says you need to find her sister in a different unrelated dungeon and you get a nice quest marker directing you there. In bg3 you find a burning building and if you make a skill check you might save the last survivor, then you can extort them for money or just kill them if you really want, then they mention their sister is trapped, you can ignore it or promise help or promise more of a reward, then you get a journal entry describing where you're currently at in that quest and it's up to you how you handle it from there.