Central 1 Java Developer interview questions

first HR called me for some questions including some java questions, such as what is difference between interface and abstract, then onsite interviewe. First, I was asked to introduce myself, then based on my previous work, interviewer asked me to draw the architecture diagram of the system I worked on the board. And he asked some questions about the architecture, such as what was the benefit to design that way. Too bad, the design was there 10+ years ago, I just worked under this whole frame. Also because the system I worked on is specialized, a lot of part such as connecting with MQ done by other teams, I could not answer.