Results Summary Component with JS Webpack and SCSS

Even though I do use ChatGPT, Stack Overflow, documentations, listed below are the things I still struggle the most with. Mostly, because there are just so many options and the answer doesn't always seem to be clear enough.

What is the best way to write your CSS ? The nested syntax I used or are there better alternatives. Of course, I know it probably depends, but how did I do when you take a look at my CSS, what should I improve ?

What about the responsive design ? You can use vmin/vmax, rem/em, %, vw/vh. How do you choose when to use which and why ? I always burn a lot of time at that.

I mixed using CSS Grid and Flebox, did I use it correctly or is it an overkill ?

Thank you very much!