Code should be your #1 priority.
What version of bootcamp are you in?
Nowadays, the vast majority of engineers are in the 2nd bucket (I'm not even sure if the 1st exists anymore). In this case, your goal is just to go through the bootcamp tasks as fast as possible to familiarize yourself with Meta's codebase and overall engineering culture.
There's really no way to prepare for bootcamp if you haven't started at the company yet. Meta's culture and tech stack are so unique that you can't exactly train at home.
How to make a good first impression?
Follow the advice here:
Yes, I'm in the 2nd bucket. I know my team/manager. Thanks both!