How Do Logins Work: A Developer's Perspective
There are two general ways that we accomplish logins: an ajax call from the app (using an Angular promise) or inside an iframe that contain a login form from WordPress.
From the App
When the login modal from the app is used, an ajax call to the AppPresser core plugin occurs. It is accessing the 'wp_ajax_nopriv_apppajaxlogin' hook in that plugin. That hook logs in the user and sends the user's details back to the app in the ajax response.
The ajax request to WordPress
The response to the app after login from WordPress
From WordPress iframe
If using login from the app, see the how to troubleshoot your login using the Postman tool.