It builds the world instead of searching for it. Where structures go depends only on part of the seed, so that part is worked out first; the rest is turned until the biomes fit. It runs Minecraft's own world generation in your browser, so the game agrees with every seed it gives you.
In Java Edition, yes. Typing words as a seed turns them into a number, and that can be run backwards: you get your name and a few letters, and typing exactly that makes the world.
Some wishes almost never happen together — an ocean monument and a woodland mansion right at spawn, say. Pick fewer, or choose a wider distance.
On Java Edition, type /seed in chat (it needs cheats on, or operator rights on a server). On Bedrock, open the world's settings: the seed is listed under Game.
