O I’m bit confused as the link drives me on the first problem of the 4.
So first which problem do you not fully understand? The 4th?
If so this problem ask you to close the open door by which white may enter your house (territory).
After closing if white start to try something by playing moves inside this territory, black will have easy answers leading to always capture the intruder.
Go is more as just a logical game. There is aesthetic and intuition involved too (50% maybe) Even if it’s far away for being obvious, it’s strangely the best weapons that a beginner has, as he can almost read nothing.