EDIT:
Ok i might have missed something - in an outgoing transaction .. do i specify which incoming transaction is used to "spend" my btc?
So in this case A and B give a btc to C - C now has two btc which he can spend. If C now gives D a bitcoin - does C specify in the transaction WHICH transaction(s) he uses as input to give D the btc? And is this probably transparent in the blockchain? That would ofcourse change how i have to think about the whole picture.
Yes, in an outgoing transaction you specify the inputs. This is usually done automatically by your wallet software. However, if for example, you enable the Coin Control feature of Bitcoin Core, you would be able to explicitly specify which inputs are used for your outgoing transaction.