What problems do frontend middle developers solve in Software Design?

Question

Software design

Hello dear friends!

Background :
Mastered React.JS and learned little by little different aspects of Software Design: SOLID, clean code, proper testing, code smells.
I haven't practiced much in Software Design, there is uncertainty. And one can say that there is no commercial experience in this field.
I plan to write a portfolio of middle frontend developer so that the project consists in solving problems close to real ones.

Questions :
1. What difficult problems do middle frontend developers solve regarding Software Design?
Interested, maybe I'm not a middle developer yet, and only junior.
Something like, "you need a confident ability to refactor according to https://www.litres.ru/martin-fauler/refaktoring-ko... "
For other hard and soft skills, I know.
2. Do you need most of all the details written in Software Design books?

Answer 1

I’m generally dumb, and I have orders of magnitude less experience, but it seems to me that it’s enough to write a large and complex project. I think that complexity can drag the same Software Design along with it.