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hi vapvarun and thank you for replying back (: ,

i tried to write a code to override the main template – and i make the script run as a plugin
here what i did

<?php
/*
Plugin Name: PAGTEM
Plugin URI: https://www.example.com
Version: 0.0.1
Author: NULL
Author URI: https://www.example.com
*/

class PageTemplate {

	/**
	 * A reference to an instance of this class.
	 */
	private static $instance;

	/**
	 * The array of templates that this plugin tracks.
	 */
	protected $templates;

	/**
	 * Returns an instance of this class.
	 */
	public static function get_instance() {

		if ( null == self::$instance ) {
			self::$instance = new PageTemplate();
		}

		return self::$instance;

	}

	/**
	 * Initializes the plugin by setting filters and administration functions.
	 */
	private function __construct() {

		$this->templates = array();

		// Add a filter to the attributes metabox to inject template into the cache.
		if ( version_compare( floatval( get_bloginfo( 'version' ) ), '4.7', '<' ) ) {

			// 4.6 and older
			add_filter(
				'page_attributes_dropdown_pages_args',
				array( $this, 'register_project_templates' )
			);

		} else {

			add_filter(
				'theme_page_templates', array( $this, 'add_new_template' )
			);

		}

		// Add a filter to the save post to inject out template into the page cache
		add_filter(
			'wp_insert_post_data',
			array( $this, 'register_project_templates' )
		);

		// template assigned and return it's path
		add_filter(
			'template_include',
			array( $this, 'view_project_template')
		);

		$this->templates = array(	
			'FullC.php' => 'Full Canvas',
			// this file exist in the same folder as the plugin like that 

		/* Plugin Template ( Folder ) ========
		 		  |
		 		  |
		 		  |
		 	Index.php (file)
		 	FullC.php (file) 
	================			*/

		);

	}

	/**
	 * Adds template to the page dropdown for v4.7+
	 *
	 */
	public function add_new_template( $posts_templates ) {
		$posts_templates = array_merge( $posts_templates, $this->templates );
		return $posts_templates;
	}

	/**
	 * Adds our template to the pages cache in order to trick WordPress
	 * into thinking the template file exists where it doens't really exist.
	 */
	public function register_project_templates( $atts ) {

		// Create the key used for the themes cache
		$cache_key = 'page_templates-' . md5( get_theme_root() . '/' . get_stylesheet() );

		// Retrieve the cache list.
		// If it doesn't exist, or it's empty prepare an array
		$templates = wp_get_theme()->get_page_templates();
		if ( empty( $templates ) ) {
			$templates = array();
		}

		// New cache, therefore remove the old one
		wp_cache_delete( $cache_key , 'themes');

		$templates = array_merge( $templates, $this->templates );

		// Add the modified cache to allow WordPress to pick it up for listing
		// available templates
		wp_cache_add( $cache_key, $templates, 'themes', 1800 );

		return $atts;

	}

	/**
	 * Checks if the template is assigned to the page
	 */
	public function view_project_template( $template ) {
		// Return the search template if we're searching (instead of the template for the first result)
		if ( is_search() ) {
			return $template;
		}

		global $post;

		// Return template if post is empty
		if ( ! $post ) {
			return $template;
		}

		// Return default template
		if ( ! isset( $this->templates[get_post_meta(
			$post->ID, '_wp_page_template', true
		)] ) ) {
			return $template;
		}

		// Allows filtering of file path
		$filepath = apply_filters( 'page_templater_plugin_dir_path', plugin_dir_path( __FILE__ ) );

		$file =  $filepath . get_post_meta(
			$post->ID, '_wp_page_template', true
		);

		// check if the file exist first
		if ( file_exists( $file ) ) {
			return $file;
		} else {
			echo $file;
		}

		// Return template
		return $template;

	}

}
add_action( 'plugins_loaded', array( 'PageTemplate', 'get_instance' ) );

now if you go to
Pages > Edit xor Add New Page > Page Attributes > A New Section Called Templates > I Click On the Full Canvas > [UPDATED]

Nothing Changed (:
Why’s Is That – Is There’s Wrong With The Code Or The Buddypress V that i use ?

i use ==> Version 6.3.0 | By The BuddyPress Community