Hey there @nigmalab!
So basically the core reason why wo do sign up / login popup instead of handling everything in the app is security concern. You can read more about various authorisation flows used by mobile apps or web apps here:
Unfortunately when it comes to Xamarin we still do not offer much of a content / quickstarts or articles regarding that. The only ting for now that we have and you can make use of is the repo that is maintained by community devs. You can find it here: