Website using html, scss, mobile first workflow

i'm open to feedback and recommendations.

I would like to know in which cases it is useful to use <ul> in the html semantics, since it is not very clear to me in which cases it should be used.

Another question is what is the best way to use the media queries, because I don't think it looks good to define so many media queries to optimize the page.

Thank you so much, in case you could solve these questions for me