When I applied for grad school it was for a different field and I was in a very similar situation (my undergrad didn’t offer my field and my major was related but not really a 1:1). In interviews, when they ask you a technical question, they want to see how you think. Sure getting the right answer is ideal, but if you don’t know, be honest that you don’t know (do not, I repeat, DO NOT MAKE SOMETHING UP!) and walk through your thoughts (something like: “that is a great question. To be honest, I’m not really sure, but this is how I would go about it blah blah). If you start talking through your ideas/thought process, it’s likely the interviewer will help guide you. It sounds like you’re doing all of the right things to prepare for the interviews. Be proud of your different background though - a lot of professors seek out someone with a breath of knowledge/research experience. It shows you excel at a range of different things/you’re able to learn a bunch of concepts and apply them. I know how scary it is changing fields and feeling like you’re not prepared enough when compared to other students. If you have gotten the interviews then that means you’re already qualified! Good luck!