So this "testnet" is not really a network or a special BlockChain view.
Is is just a special BlockChain server.
The Sender has manually to add the MetaData and Receiver has manually to check this special server for reading the MetaData.
If the receiver does not know that the sender has added something then he will never read it.
Correct?
No, the testnet is a network of nodes that also keep a decentralized blockchain ledger, just like the main net.
The biggest difference between the testnet and main net is that at the very beginning, everybody agreed that testnet coins are worthless, and that the testnet can be 'reset' at any point in time... There are some other differences, but overall, the testnet behaves like the mainnet. So, you can use the testnet for testing without risking real money... If something works on the testnet, it'll most probably work on the mainnet