My question is: Where is this 12,5 new BTC are coming from?
It is not "coming from" anywhere.
The rules of bitcoin allow the miner to include a single special transaction in each block (often called the "coinbase" transaction or the "generation" transaction) assigning 12.5 BTC of value that comes from nowhere (plus the transaction fees of the transactions in the block) anywhere they like when they create the block.
The BTC is the new block itself?
The BTC is the value of the outputs in the special transaction.
