Hi there!
There is extensive documentation provided on how to make plugins
Here is really simple and friendly guide on how to start creating them
Here you have technical details on each and every event
/ hook
available in Grav Core
And finaly if you need hooks for user authentication you should take a look at
grav-plugin-login
on GitHub
– as it is most popular authentication solution for Grav, and it provides a whole lot of related hooks, like:
onUserLogin
onUserLoginRegistered
And many more!
Also, you may want to take a look at Grav API as it contains definition of almost every object and class used in Core, as well as many for other plugins.
Hope you’ll find in the above documents all the guidelines you need, and I really, really hope that when your plugin is redady you will show it to us!
Cheers!