The interview was a panel of three current employees (your future boss, your future coworker, plus one other senior-level engineer that may/may not work with you). They ask you the typical questions about why should we hire you, tell us about yourself, etc. Then the technical parts begin and they try to benchmark your engineering knowledge to see if you are E1, E2, E3... It is challenging because it feels like they are trying to "break" you.