#3
its a key change. key changes in music give the song a lift or make it darker, depending on how you want it to sound. this key change made the song happier, uplifting it in a sense. thats whats going on

ps. key changes can also build/release tension. //the more you know!
#4
The key appears to be modulating to A major.
Ka pu te ruha ka hao te rangatahi.