How to debug, test php? How to test html with .html removed (.htaccess)?

I'm wondering how professional web developers debug, test, and deploy websites with some specific questions.

#1 PHP

For a big site like amazon.com, I imagine they don't upload to the server to test php code. So how do they do it?

Is there a software or IDE I could use to test php locally?

#2 debugging without .html .php extension?

In addition, most sites remove .html and .php in url I'm also doing this by editing .htaccess

# remove .html .php
Options -MultiViews
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule ^([^\.]+)$ $1.html [NC,L]

But when I do this, I need to reference other html and php files without adding the extension

<a href="about">About</a>
<a href="contact">Contact</a>

So then I can't debug locally, since links won't work without uploading

How can I debug locally with this specific .htaccess?

How do other sites do it?

Any tips or general practices on website debugging, deployment, and testing would be appreciated.