Junior solution with BEM/FlexBox/JS class manipulation. ADVICE NEEDED!

It is my first junior level project and it took me longer than I thought it would. I have couple of questions:

1. Main navigation arrows are not 100% working the way it should. Where is my logical mistake working with pseudo classes in CSS?

2. Similar problem with pop up navigation window - it doesn't disappear, if I click away. How to fix that?

3. I still have a scroll on x-axis that I would like not to have. Why it is still there?

4. How to position photos closer to a look of the actual challange?

Thank you for the advices in advance - still looking for a coding pal, that I could learn to code togeather with a help and a review of a code.

Regards, Jaronimas