So I appeared for the first round at FB London online.
For any of those like me who are looking to understand more about iOS interviews

So, I was asked one iOS based general question on design patterns then 2 coding questions
First coding question was about string [Easy leetcode]
Second one was traversing a tree [Medium, this one on LC is a rarely asked question in last 6 months]. This question was tree traversing but linked to the iOS framework. So its important to have knowledge with iOS api's.