To get the best offer, you have to have leverage in negotiations. And the best leverage is the competing offer(s).
Being in the interview process with other companies already makes you a more valuable candidate. Every other company in your pipeline decreases the chances you're going to accept the current offer.
Ultimately, you want to get into the position where you have offers from 2+ companies you're ready to work for.
This is a situation with no bad outcomes. It will also make you comfortable asking for a better offer and not being afraid to be rejected, as you have backup options.
To build this leverage, you have to plan. Our entire Work for FANG guide will help you to prepare and run the interview process with multiple companies in parallel