Life In 19x19

There is no point for White to invade that territory. If a sequence

:w1: :b2: :w3: :b4: :w5: :b6: follows and the three white stones are dead, then Black played 3 times inside his territory (-3 points) but gets 3 extra prisoners (+3 points) so Black still wins.

I understand. Black just continues ...

Thank you all for your answers so far, they are indeed very helpful. I understand, that most players agree on the end of the game and captured groups, but for me it is important to understand when this is the case, based on the theoretical progression of the game, if it was played out. Otherwise ...

So if AGA rules specifically address this, can it be said, that certain (probably more classic) rules do not sufficiently address this (or not at all)?
To me, the AGA solution seems plausible, but I Do not understand the virtual play solution yet. When would that Beginn? The moment both players pass ...

I am relatively new ot Go, and have only played a couple of games. My biggest issue is with scoring (it's the same problem for all scoring systems I have come in contact with).
The problem I face is this:
How to evaluate a position in my head (scoring wise) if the game ends on "agreement"?
Or rather ...