In terms of how to prep for interviews, the best thing to do is to research these topics yourself, and work on personal projects that use them.
For example, my first software job was similar, we didn't engage in any good practices and they were hard to implement. So I worked on private projects, where I could do what I want and had the time. In those projects I would properly plan things, I would set up the src control properly, I would test all my code, I would comment code and try to make it understandable, reusable and scalable, etc.. So when it came time to talk about these best practices in interviews, I had some decent knowledge and experience in them, even if I hadn't been exposed to them at my actual job.
I tend to find that interviewers don't want specific examples of these practices from your current job, they just want to know that you're aware of them and what they involve. You may be hindered from being exposed to them in your job, but nothing stops you researching and using them outside those hours. It'll definitely be worth the time, career wise. And personal projects which exhibit these best practices are great for your portfolio, even if they're small ones.
If they absolutely press super hard for current job examples, then I personally would just say that your current work doesn't really do it, so you took the effort to learn/practice them yourself. That shows initiative and may provide them with extra context as to why you're looking elsewhere.