Interviewing (especially system design) is a skill you have to learn these days. It's also a performance. You can complain about how flawed it is all day, and believe me I do (I'm a staff FAANG swe), but you gotta play the game. Knowing how to methodically break down the problem into smaller components and be able to confidently present the information in a manner that's easy to follow/collaborate is actually a real on-the-job skill of any experienced engineer. You have to be able to demonstrate this in an interview. The people at interviewing.io are legit and this guide is a great prep tool