https://www.klaskolaw.com/faq/green-card-quota-backlogs/ says:

“As long as you have a pending I-485 application at USCIS, you are eligible to apply for and receive EAD and AP documents.”

http://www.alllaw.com/articles/nolo/us-immigration/maintaining-legal-h-1b-status-while-waiting-green-card.html explains the basics:

Instead of extending your H-1B, when you file your green card application you can also file for work authorization (Form I-765, referred to as an employment authorization document or EAD) and travel authorization (Form I-131, referred to as Advance Parole or AP). These documents will allow you to work in and travel outside of the U.S. while your I-485 is pending.

So i-485 is a process, while EAD is a document you rely on while your i-485 is pending.

• With the EAD card, you can work for any employer.
• 180 days into the i-485 process, you are allowed to change employer.

These are major benefits of early filing of i-485. You can file your i-485 when you notice the moving DateForFiling cutoff becomes Earlier than your PD. See GC priority date – 2 cutoff dates on visa bulletin

–https://www.klaskolaw.com/faq/green-card-quota-backlogs/ echos that

In order to take advantage of AC21 portability, the I-485 must be pending over 180 days.