Flutter, Laravel Sanctum and Socialite, introduction

<tl;dr>This story contains several posts, this post is an introduction to the requirements, to the problem and to the players. This post doesn’t contain any Dart code for the Flutter frontend or php for Sanctum/Socialite backend. </tl;dr> The requirements I’m trying to meet Frontend application that is initially loaded from an another domain than whereJatka lukemista ”Flutter, Laravel Sanctum and Socialite, introduction”