Not quite 'always' but over 4 billion years.
The Moon, Sun and planets all appear in a strip of sky called the ecliptic. It is a plane defined by the plane of the Earth's orbit and the Sun is always on the ecliptic, while the Moon and planets stay close to it.
planets stay in the sky due to the pull of gravity
Even though it's not apparent to the naked eye, the light from the planet covers some area of sky, so some of it always manages to get through.
You can't
In our solar system, the three planets that are generally visible to the naked eye in the night sky are Venus, Mars, and Jupiter. These planets can often be seen without the aid of a telescope or binoculars.
The zodiac is the "belt" around the sky within which the sun is always located. The moon and planets also always remain in the zodiac or close to it. That's the reason that this part of the sky was long thought to have some special significance with regard to life on earth.
The planets shine because they reflect sunlight.
The planets most certainly ARE visible in the northern hemisphere.
because some of the planets reflect lights from the sun like Venus.
Galaxy
No. We know what the stars are. They are not planets. They are distant suns, many of which do have undiscovered planets.
Ancients distinguished between planets and stars in the night sky by observing that planets move relative to the fixed background of stars, while stars maintain their positions.