Let's say I have 20 bitcoins in address A. I sent 5 bitcoins from address A to address B. Why is that the remaining bitcoins got sent to address C that I never used or knew before.
I still get access to the remaining 15 bitcoins using the private key of A, but it doesn't show anymore in recipient address A. This is very confusing and would appreciate if someone would clarify.
Also, when I need to receive any bitcoins, which I address should I use? Original address A or address C?