What to do with case they are multiple unspent ones?
I already said it, you get a list of UTXOs and you can spend as many of them as you want.
Each bitcoin transaction has
an array of inputs and
an array of outputs that have to have at least one item in them (one input and one output) but you can create a transaction with for example 10 inputs and 4 outputs.
You don't need a watch only wallet first, it is an option to maximize security.
Does that mean with just the - private key - I can create a raw transaction without getting any data from the Bitcoin network?
No.
A watch-only wallet is a wallet that does NOT contain any private keys. It is created using the addresses (or technically could be with public keys too). It provides security since your private keys remain offline.
But you still need to construct a transaction knowing the UTXOs you can spend and you can only find them by knowing the address and searching the bitcoin blockchain.