How to have a separate page? What should be the link to that page?

by

Method 1 (Treating it as a WordPress page):

1) Add following on the top of projects.php

/*
Template Name: Projects
*/

2) Create a page from wordpress and select ‘Projects’ as its template from the right menu

3) Now you have a project page. Use its URL in anchor like you use other wordpress pages’ URL.

Method 2 (Not treating it as a wordpress page)

<a href="https://wordpress.stackexchange.com/questions/219791/<?php echo get_stylesheet_directory_uri(); ?>/projects.php">Link Text</a>

For more info about get_stylesheet_directory_uri() function:
https://codex.wordpress.org/Function_Reference/get_stylesheet_directory_uri