Minecraft Seeds and World Generation — What the Number Really Does

Almost always a version mismatch. World generation changes between updates, sometimes subtly and sometimes dramatically, and any change to the generation algorithm means the same seed produces different terrain. A seed from a video recorded two versions ago will not reproduce that world on the current version. The edition matters too — Java and Bedrock use different code entirely.

No. They use separate generation implementations, so the same number produces genuinely different worlds. Some structures land in similar places by coincidence in certain seeds, which creates the impression of partial compatibility, but the terrain is not the same. Always check which edition a seed was found on before using it.

Not for terrain that has already generated, since those chunks are saved to disk. Changing the seed in the world files affects only chunks generated afterwards, which produces a visible and usually ugly seam where the old and new generation meet. If you want a different world, make a new one.

In single player with cheats enabled, the locate command finds the nearest instance of a biome or structure and gives you the coordinates. On servers this usually requires permissions. Third-party seed mapping tools also exist that render an entire world map from a seed and version, and they are the fastest way to plan a base location before you start playing.

Only relative to what you want from it. A seed with no village within several thousand blocks is difficult for an early game but interesting for a survival challenge. What people usually mean by a good seed is convenient starting resources near spawn, and that is a preference rather than a property of the world itself.