What Happens to the Nonce Once a Block Is Successfully Mined?

Once a miner finds a nonce that results in a valid block hash (meeting the difficulty target), that specific nonce is permanently recorded and broadcasted as part of the new block header. It is the "proof" that the work was done.

The nonce is then validated by all other nodes on the network to confirm the block's legitimacy before they accept it and begin mining the next block.