This is exactly the problem I have too. I get interview questions like "You have to design xyz system, such that junior devs can start building it. How would you do it?"

I find it very tough to answer them unless I have personally faced the problem or if I have read extensively about it.

Sometimes I think that startup experience doesn't prepare you for this kinda stuff. In the early stages, it's all about "Fuck it, just ship it." So design and all go out the window. This is what is happening in my current company too.