Negotiable: RSU grant, signing bonus, start date, team preference (during team shopping).
Partially negotiable: Base salary (narrow band per level, \10$20$K flexibility).
Not negotiable: Level (determined by interview), bonus target percentage (set by level), vesting schedule.
The team shopping phase is unique to Meta. You can talk to multiple teams and choose the one that fits best. This is not available at Google (team match before offer) or Amazon (team assigned at offer).