Whenever the game has to generate a new world, it calls upon an algorithm known as Perlin noise. This algorithm outputs a pseudo-random value that is then used to determine the characteristics and features of the world. However, the algorithm always outputs the same value each time for a constant starting point (seed). Thus, the same seed will generate the same terrain every time.

A world's seed is set when that world is created. By default, it is decided automatically, but it can also be set manually. Setting and reusing a seed from one world generates the same world. Either a number or a word/phrase can be used, including negatives. If a word/phrase is used, it is converted into a 32-bit integer.

Whenever the world generation algorithm is updated (usually by adding new biomes to the game), the same seed no longer generates the same terrain. If the seed or generator changes in a saved world, new chunks are based on the new seed and no longer match those from the old seed.

Because seeds are simply random values read into an algorithm and not actually names of different worlds, using a certain seed does not result in a world with any relevance to the value of that seed. For instance, using a biome name as the seed does not necessarily result in the creation of a world with primarily that biome, nor does it spawn the player within the said biome.

This is one of the best 1.18 seeds I've come across. It has a really large swamp with 2 witch huts very close together. There are pumpkins scattered around the swamp. There are 5 other biomes for a total of 6, swamp, jungle, badlands, savannah, desert, plains. There is a jungle temple, and a desert village.

This video was recorded on my xbox one x in the Minecraft Bedrock Edition beta. It will work in the 1.18 caves and cliffs update version.