Who decides what the next, correct Nonce is? As I understand finding the correct Nonce is the basis when hashing transactions for Proof of Work.

How and where is this decided in the ecosystem what the next Nonce is?

In my limited understanding this part needs to be centralized somehow, that every node is searching for the same, correct one. At the same time this might be an important security aspect.
Each node is searching for a different set of Nonces, actually.
This is because each node collects randomly a bunch of transactions that will form a block. So each block that the nodes are mining is different. On top of that, there are many Nonces that could make a correct form of hash.

The only thing here that matters is:


all the data inside the block seem to be correct (correct inputs, with correct and confirmed transactions, with their correct associated hashes, etc.).
Nonce + Block gives you a hash smaller than a predetermined number (known by all the nodes).