Learning how to optimize a game for mobiles is something that is hard to teach.
Whatever you will read here now will be meaningless unless you find yourself in similar situations.
iOS and Android devices are unfortunately weak and this will force you be a stronger developer.
While optimizing a program should not be done at the beginning of the development cycle,
you will encounter easy to follow practises that will prevent you for doing the same mistakes again and again.
Optimizing a game is half experience half art. You have to learn the shortcuts and second ways of every Import option, Unity behaviour and Line of code you write.