The seed of the world does not change if you change the world. If you recreate the world using the original seed and the same world generation parameters, the world will be pristine and exactly the same as when you started, with no changes you've made to the world persisting. If you wish to preserve your changes, you'll need to use a tool like WorldEdit or Structure Blocks to transfer your buildings over to the pristine world.
The seed does not store anything, and it doesn't change when the world changes. It is literally just a starting value for the random number generator used for world generation. So if you use the same seed for two worlds, the worlds will generate the same.