How can I log a user into my mobile app via Shopify's API?

[M_tech]

hello ! 

I'm building a flutter app where I've done app's work via admin API like fetch all products and listing collections etc. I'm also able to create orders add tags and shipping address and billing address etc. as well. The only problem I'm facing at this point is how to log a user in that's registered on Shopify store. I did looked into customer's API in documentation but there was only the API for creating a user, I didn't found any where it was written to login a user and accepts username and password. So please help me I'm stuck here. I'm not that expert of Shopify. I'm also confused with the flow of Shopify as there are so many things some say use Admin API some say use graphQL some suggest use storefront API which is more confusing than the other two

thank you for your help in advance