I have around 6 years of work experience. In my workplace I generally do a lot of system design and architecture but when it comes to interview, I always feel lost with the design questions. I find it hard to getting started with. For example if its a system design, I generally ask questions like scale and users but I get confused if I should start by defining the interface and database behind it. Also how much granulity should I go into, should I tell about the tables and the different type of apis I am going to need. Also should I go into the data structure in the system design which I am going to use.

Object oriented design are even more difficult. Here in Leetcode most of the questions revolve around coming with the data structure and algorithm for the design questions and we are given an interface/structure of the class to begin with but in the interview I generally find it hard to come up with the intial class structure. I have tried to look into a several design questions online and most of the time feel I am prepared for taking the design questions in the interview but in the end screw it up while getting started. Any help on this or reference will be appreciated.