Meta uses an E-level system for engineers:
E3 (Entry): New grads. - year of experience.
E4 (Mid): - years. This is where most external hires land.
E5 (Senior): - years. System design round added. Higher coding bar.
E6 (Staff): + years. Requires demonstrated cross-team impact. The bar here is much higher than E5.
Your level is determined during the interview, not before. If you interview for E5 but only show E4 skills, Meta may offer you E4 instead of rejecting you.