Here is important reference/points to learn word press.

Function Reference - https://developer.wordpress.org/reference/

WordPress Coding Standards - https://make.wordpress.org/core/handbook/best-practices/coding-standards/

Data Validation - https://codex.wordpress.org/Data_Validation

Global Variables - https://codex.wordpress.org/Global_Variables

Post Types - https://codex.wordpress.org/Post_Types

Taxonomies- https://codex.wordpress.org/Taxonomies

Query Overview - https://codex.wordpress.org/Query_Overview

Plugin Developer Handbook- https://developer.wordpress.org/plugins/

Theme Developer Handbook - https://developer.wordpress.org/themes/

Detailed Overview of class WP_Query - https://codex.wordpress.org/Class_Reference/WP_Query

Custom Queries - https://codex.wordpress.org/Custom_Queries

Hooks - https://developer.wordpress.org/reference/hooks/

WP AJAX - https://codex.wordpress.org/AJAX_in_Plugins

Plugin API/Filter Reference - https://codex.wordpress.org/Plugin_API/Filter_Reference

Plugin API/Action Reference - https://codex.wordpress.org/Plugin_API/Action_Reference

Conditional Tags - https://codex.wordpress.org/Conditional_Tags

Templates - https://codex.wordpress.org/Templates

Template Hierarchy - https://developer.wordpress.org/themes/basics/template-hierarchy/