To avoid files outside WordPress, how to create a custom page?

ANSWER: Several clients often upload directories or .HTML or .PHP files outside their WordPress installation because they think its the only way to perform a certain function or design an entirely custom landing page or otherwise. This is 100% inaccurate, and can cause serious issues in regard to speed, security, and stability because these URLs are not benefiting from the security and coordination that is built into WordPress (and our Nginx rules).

Instead, simply create a new page in your WordPress i.e. “custom-landing” and then if you want a totally custom design for that page you just upload a new file under your theme’s directory called “page-custom-landing.php” and WordPress will automagically use that PHP file design as the design for the relevant URL it belongs to. Don’t forget to rename that PHP file in your theme directory in case you decide to change the slug of that page in your WordPress database.