The campaign structure is the clearest thing about Diablo: Unbound, and the loot economy is where most of the decisions live.
Each act is three dungeon levels, and every act ends with a boss. Budget 60-90 minutes per dungeon and 3-4.5 hours per act - which means an act is a session, not a single sitting. If you have less time, play a dungeon rather than attempting an act.
Between dungeons you get a Town Phase to spend XP on skills, sort and equip loot, and respec if your build went somewhere unhelpful. This is the only window in which you can change your loadout - there is no second look before the next dungeon, so do not skip it.
Loot is randomised, and it comes from four places: chests, Elites, side quests and kill streaks. Bosses hold the best gear. That combination means skipping side content has a real cost - the reward curve is not evenly spread - while farming kill streaks is the most repeatable way to build XP and gear between bosses.
Three enemy categories are confirmed: Khazra, Demons and Undead, plus a randomised mid-tier Elite tier that drops loot. The specific boss roster has not been announced. Each boss has unique attacks, a lair and multiple combat stages, and holds the best gear. Canonical Diablo bosses such as the Butcher or Maghda have not been confirmed as appearing - treat any list naming them as speculation.
Torment mode is the endgame option: fight any boss "regardless of their tier, in its ultimate form", scaled for endgame characters, with extra loot and extra event cards in play. Because it is scaled for endgame characters, running it early stalls progression rather than rewarding it - save it for after the third act.
The design is co-op-first for one to four players, so solo sessions are longer and grindier. Your main compensation lever is the Town Phase - aggressive loot management and respec. If you are playing alone, budget more Town Phase time than a group would, and use it to keep your board coherent.