Wordpress plugin filter/action documentation for v5.x?

On version 5.x there doesn’t seem to be any apply_filters / do_action calls inside code. I’m guessing there is a way to modify this (didn’t have time to dwell into src just yet, but I see hints that there is at least maybe some kind of architecture present for these…). But I can’t seem to find any documentation for this. Is there any? Or is the idea really that the plugin is what it is and all the modifications should be done through SDK?

As an additional question, do Auth0 people have something against WP and PHP? (why aren’t wordpress and php allowed tags in Help category?)