There is no default temperature. The recipe is telling you the temperature: 350F. You preheat the oven to 350F so that it's already fully hot when you put the cake in, and then you leave it at 350F to bake the cake.
The whole point of preheating is to have the oven already at the temperature you want to bake at. If you preheated then changed the temperature, then you wouldn't have preheated properly: the oven would be at the wrong temperature.
If a recipe actually wanted you to change the temperature, it would explicitly say so, something along the lines of "preheat the oven to 550F, put the bread in, then reduce the temperature to 500F". That'd cause it to be baked at closer to 550F at first, then closer to 500F later once the oven cools down. But that's not something that comes up much; most things just get baked at a constant temperature.
Bottom line, just do what the recipe actually says to! If it doesn't say to change the temperature, don't.