how to make one log in for my iOS app and website?

Hi developers , I am building my first projects. I like to use gmail (auth2.0) for my log in. My backend is in Django. I can implement sing up and sign in for my website but I don't know how use the same sign up and log in for my iOS app (Hopefully latter android app) Anyone can guide me into right direction ? Do I need to make one restAPI with Django and that connect with how many UI that I want? Which docs should I check ?