Question
how does a seed plant survive if tall buildings block out sunlight and sunlight is available 6 meters to 10 meters above ground level?

Answer
Hi Madison,
thanx for your question.  Are you talking about a weed growing amongst the concrete, maybe in a crack in a very urban environment?  If so, weeds are very, very adaptable oranisms and can often survive with minimal soil, water and light, growing in shaded areas, in cracks and crevices of rocks and concrete.  Other plants can survive with a minimal amount of sunlight (such as plants grown in the urban canyons of New York and other large cities because there is a short window where sunlight is available to the plant even when it is partial, dappled or reflective sunlight.  There are a lot of plants that are adapted to shade but not a complete lack of light.  During the day, there is reflective light which many plants can use to survive.  All terrestrial plants that I know of, and I'm talking about complex structures not microscopic structures require light for photosynthesis.  No photosynthesis, no life.  I hope this helps and bear in mind that I am not a botanist so my understanding of this subject is purely at an amateur level.  

Tom