This is an OOP based WordPress plugin boilerplate with namespaces & composer auto load classes
SPB_PLUGIN_PATH -> Used to get path of the plugin directory
SPB_PLUGIN_URL -> Used to get url of the plugin directory
SPB_PREFIX -> Used to print prefix of the plugin (Recommended for PostTypes, Post Metas, Database Tables etc) spb_
SPB_PLUGIN_NAME -> Used to print name of the plugin to use as key - smartx_plugin_boilerplate
SPB_PLUGIN_TITLE -> Used to print title of the plugin ( Human Friendly Version of a name ) - Smartx Plugin Boilerplate
SPB_TEXT_DOMAIN -> Used to get Text Domain of a plugin for used in language translations - smartx-plugin-boilerplate
SPB_AJAX_URL -> Used to get ajax url in wordpress - site_url/wp-admin/admin-ajax.php
In includes/Initialize.php you can find addActions() method. use $this->loader->addAction('your_action',$class,'yourCallbackMethod'); to load your action and create action callback method in includes/callbacks/Actions.php
In includes/Initialize.php you can find addAjaxActions() method. use $this->loader->addAction('your_wp_action',$class,'yourCallbackMethod'); to load your ajax action and create ajax action callback method in includes/callbacks/Ajax.php
In includes/Initialize.php you can find addFilters() method. use $this->loader->addFilter('your_filter',$class,'yourCallbackMethod'); to load your filter and create filter callback method in includes/callbacks/Filters.php
In includes/Initialize.php you can find addShortcodes() method. use $this->loader->addShortcode('your_shortcode',$class,'yourCallbackMethod'); to load your shortcode and create shortcode callback method in includes/callbacks/Shortcodes.php
In includes/callbacks/Styles_Scripts.php you can find frontendStyles() method to add stylesheet on frontend and backendStyles() method to add stylesheet on backend.
In includes/callbacks/Styles_Scripts.php you can find frontendScripts() method to add script on frontend and backendScripts() method to add script on backend.