How to use IOS built-in font like Helvetica

Hi there, After many research I still have no idea how to use IOS built-in font. For example I can create an "Hello World" app in Xcode directly and draw a text with different language,font and size. But with unity, things became more complex. The Unity Manual suggest importing font by add .ttf to asset folder. I'm not sure doing so would enlarge the built size. I'll be grateful if somebody would helpme found the best practice to use iOS native font.