Seed

Minecraft seeds are values made up of numbers (including negative or positive integers) that are used as the basis for generating every Minecraft world.

Time
Seeds are made by current time, but there is no proof, if this theory is true.

Name of world and time
In this theory, world is made by name of world and time. This theory is made on fact, that letters are translated into binary, which are translated into numbers.

Random
This theory says, that seeds are just randomly picked number.

Number of seeds
Minecraft:Java Edition seeds are 64-bit, but in Minecraft:Bedrock Edition are only 32-bit.