How can I authenticate a user from a mobile app

I was wondering, if a user has been authenticated using wasp auth from the website, how can I authenticate that user from my mobile app? Is there any sort of API that I need to use for it?

Sorry if my question is too noob. Web seems very vast and overwhelming, I am just looking for something easy.

So far I have been using firebase to authenticate users and do the subscription through App Store. But I need to have a web authentication now