how wordpress call own functions?

Hi…
I’m new to wordpress and want to know how it works!!
At this moment i started read wordpress functions from here:
https://codex.www.ads-software.com/Function_Reference
The first function is get_adjacent_post and it located in wp-includes/link-template.php
here is get_adjacent_post function codes:

function get_adjacent_post( $in_same_term = false, $excluded_terms = '', $previous = true, $taxonomy = 'category' ) {
	global $wpdb;

	if ( ( ! $post = get_post() ) || ! taxonomy_exists( $taxonomy ) )
		return null;

	$current_post_date = $post->post_date;

	$join = '';
	$where = '';

	if ( $in_same_term || ! empty( $excluded_terms ) ) {
		$join = " INNER JOIN $wpdb->term_relationships AS tr ON p.ID = tr.object_id INNER JOIN $wpdb->term_taxonomy tt ON tr.term_taxonomy_id = tt.term_taxonomy_id";
		$where = $wpdb->prepare( "AND tt.taxonomy = %s", $taxonomy );

		if ( ! empty( $excluded_terms ) && ! is_array( $excluded_terms ) ) {
			// back-compat, $excluded_terms used to be $excluded_terms with IDs separated by " and "
			if ( false !== strpos( $excluded_terms, ' and ' ) ) {
				_deprecated_argument( __FUNCTION__, '3.3', sprintf( __( 'Use commas instead of %s to separate excluded terms.' ), "'and'" ) );
				$excluded_terms = explode( ' and ', $excluded_terms );
			} else {
				$excluded_terms = explode( ',', $excluded_terms );
			}

			$excluded_terms = array_map( 'intval', $excluded_terms );
		}

		if ( $in_same_term ) {
			if ( ! is_object_in_taxonomy( $post->post_type, $taxonomy ) )
				return '';
			$term_array = wp_get_object_terms( $post->ID, $taxonomy, array( 'fields' => 'ids' ) );

			// Remove any exclusions from the term array to include.
			$term_array = array_diff( $term_array, (array) $excluded_terms );
			$term_array = array_map( 'intval', $term_array );

			if ( ! $term_array || is_wp_error( $term_array ) )
				return '';

			$where .= " AND tt.term_id IN (" . implode( ',', $term_array ) . ")";
		}

		if ( ! empty( $excluded_terms ) ) {
			$where .= " AND p.ID NOT IN ( SELECT tr.object_id FROM $wpdb->term_relationships tr LEFT JOIN $wpdb->term_taxonomy tt ON (tr.term_taxonomy_id = tt.term_taxonomy_id) WHERE tt.term_id IN (" . implode( $excluded_terms, ',' ) . ') )';
		}
	}

	$adjacent = $previous ? 'previous' : 'next';
	$op = $previous ? '<' : '>';
	$order = $previous ? 'DESC' : 'ASC';

	/**
	 * Filter the JOIN clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, <code>$adjacent</code>, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since 2.5.0
	 *
	 * @param string $join           The JOIN clause in the SQL.
	 * @param bool   $in_same_term   Whether post should be in a same taxonomy term.
	 * @param array  $excluded_terms Array of excluded term IDs.
	 */
	$join  = apply_filters( "get_{$adjacent}_post_join", $join, $in_same_term, $excluded_terms );

	/**
	 * Filter the WHERE clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, <code>$adjacent</code>, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since 2.5.0
	 *
	 * @param string $where          The <code>WHERE</code> clause in the SQL.
	 * @param bool   $in_same_term   Whether post should be in a same taxonomy term.
	 * @param array  $excluded_terms Array of excluded term IDs.
	 */
	$where = apply_filters( "get_{$adjacent}_post_where", $wpdb->prepare( "WHERE p.post_date $op %s AND p.post_type = %s AND p.post_status = 'publish' $where", $current_post_date, $post->post_type ), $in_same_term, $excluded_terms );

	/**
	 * Filter the ORDER BY clause in the SQL for an adjacent post query.
	 *
	 * The dynamic portion of the hook name, <code>$adjacent</code>, refers to the type
	 * of adjacency, 'next' or 'previous'.
	 *
	 * @since 2.5.0
	 *
	 * @param string $order_by The <code>ORDER BY</code> clause in the SQL.
	 */
	$sort  = apply_filters( "get_{$adjacent}_post_sort", "ORDER BY p.post_date $order LIMIT 1" );

	$query = "SELECT p.ID FROM $wpdb->posts AS p $join $where $sort";
	$query_key = 'adjacent_post_' . md5( $query );
	$result = wp_cache_get( $query_key, 'counts' );
	if ( false !== $result ) {
		if ( $result )
			$result = get_post( $result );
		return $result;
	}

	$result = $wpdb->get_var( $query );
	if ( null === $result )
		$result = '';

	wp_cache_set( $query_key, $result, 'counts' );

	if ( $result )
		$result = get_post( $result );

	return $result;
}

In first if(){} we called get_post() function that located in wp-includes/post.php and wp-includes/class-wp-atom-server.php
Now my question! :
Why get_post() is working?!
It’s located in (wp-includes/post.php and wp-includes/class-wp-atom-server.php) and we don’t have this syntax:

include "post.php";
OR
include "class-wp-atom-server.php";

?! ??