Responsive website using CSS Grid and JS Async Functions

I had a great time building the website and I think I did a good job, but I'm kinda unsure about some parts of it. Basically, I had difficults making the website responsive at the width of 375px since mine is kinda different from the original picture (margins, bar size). Some advice about how to solve it would be appreciated.

Also I have no idea how the JS async functions work, I just copied&pasted it in there and it just seemed to work, if somebody could explain how and why it works I would be very glad.