Sid is the key of the world's generator: the number from which the game builds terrain, biome and construction. With the same seat in the same version of the game, the world will come out the same way. You can recognize it by command. /seed:The game writes "The Generator Key: ..." and the number in the answer is copied by a click, the clue says, "Push to copy into the exchange buffer."
There is no seed on the F3 screen: in 26.x not a single debug line shows it, so you can only find it out with a command.