Answer 1
During the day the stars are still there, you just can't see them because the sky is so bright. For example, it's a lot easier to see the glow of a candle in the dark than in the light.
It is always hard to sense small changes to something that is already big to start with. For example, in a noisy room you probably can't hear a dripping faucet, but when everything else is quiet then you can hear the drip, even though the dripping hasn't gotten any louder. Its just easier to hear something soft in a silent background, than it is to hear something soft in a noisy background.
In the same way it's easier to see the dim starlight with a dark background than it is to see the same dim starlight on a bright background