Evolving thought process and design thinking

Question

I'm working as a software developer for around 8 years now and have worked with mostly companies offering outsourcing services.The challenge so far was mostly - learn the new tech, showcase through a prototype/POC and implement it in the assigned project. Over the years, technologies changed, but nature of work remained the same - javascript, jquery, webforms, silverlight, MVC, etc. But now, am trying to transition to a software consultant role and one of the feedbacks that I recieved was - "Try to think in depth, as to not just use a feature but to relate it to the most basic concept that you know." e.g. as a best practice for string concatenation in C#, we know that we should be using stringbuillder and also know why we should be using it (string immutability), but the question was - How would you implement a StringBuilder class from scratch?

 

I'm not sure how to train myself for becoming ready for design thinking and making myself ready for in-depth thinking and analysis (since most of day to day tasks never require such rigorous drill-down).

 

PS: Sorry for long question and also even though the question is not suitable for platform, I would still be happy if anybody could point to a correct forum/community where i can reach out for guidance.