This has really evolved over time. Originally, there were single-game randomizers, which became more and more sophisticated in what they could randomize. Early on, those randomizers added "logic", meaning that they ensure you can't have an impossible random seed where an item is stuck behind a point that needs that item to proceed, or a set of items that would mutually require each other.<p>At some point, people started combining games; most notably, "SMZ3" is a two-game randomizer for Super Metroid and Link to the Past (already two very popular games for randomizers), in which you're playing one SNES cartridge that has both games on it and you can step in specific doors to switch from a spot in one game to a spot in the other game.<p>Archipelago is an evolution of those multi-game randomizers, which combines the logic from each game to ensure that <i>someone</i> is always able to proceed.<p>And there are options you can choose to make things simpler; for instance, you can prevent certain key items from being very late in the game. But it's also fun to <i>not</i> do that, and end up (say) having to do half a game without an item you usually get in the first few minutes, working around the lack of that item. (For instance, playing substantial parts of a Zelda game without even a basic sword.)<p>Some randomizer modes also convert a more linear game <i>into</i> a more open-world game, by making it so you can go anywhere from nearly the beginning, though what you can <i>do</i> in those places will still be limited by the items you have.