well isent there a servival mode if so can someone tell me how to get to it i was watching videos on youtube about it would like to try so pls help and thank u for ur time

There's a mode called Survival, where you build your base and survive for a set amount of time. And then there is Skirmish Defense which is the thing I think you're looking for. You go to "Scenario Battle" , and then select a mode at the top left.

Um, Good luck on your endeavors?