Evergreen plants like holly, ivy and mistletoe are symbols of life and fertility in the face of the bleak winter months which can be a metaphor for death. They have been brought into homes in Britain in the Winter for thousands of years.
Anything with lights on are a nod to the Sun coming back to give us longer and warmer days in a new year. They are fake versions of us burning things to symbolise the Sun’s light, like the burning of the Yule Log, a custom brought to us by Scandinavian invaders and settlers.
Christmas trees were from the Holy Roman Empire, where they’d have them in the home for the same reason we had holly, ivy and mistletoe. The earlier Roman Empire had tree branches and wreaths in the home for Saturnalia.